Edward Alfred Minchin

“ We may assume, therefore, at least as a working hypothesis, that a lethal parasite is one that is new to its host, and that a harmless parasite is one long established. Since all parasites must have been new to their proper hosts at some period, recent or remote, in the history of the species, it would follow that the first commencement of parasitism would be in almost all cases a life and death struggle, as it were, between the two organisms concerned, and it is quite conceivable that the host might succumb in the struggle and so be exterminated. ”

“ The Bacteria stand sharply apart from the other forms of life, not only, in many cases, by their divergent methods of metabolism, but by morphological characteristics, such as the definite body-form limited by a distinct envelope, the absence of organs for locomotion other than the peculiar flagella, and, above all, by the lack of any differentiation of the body-protoplasm into nucleus and cytoplasm, as in all true cells of either animal or vegetable nature. ”

“ PROTOPLASM, the name given in modern biology to a substance composing, wholly or in part, all living cells, tissues or organisms of any kind, and hence regarded as the primary living substance, the physical and material basis of life. ”

“ Only when the organism becomes specialized for one or the other mode of life exclusively does it acquire such definite morphological characters that the difference between plant and animal can be used for the purpose of a natural classification, as in the higher forms of life. ”

“ In Leptolinae the actinula becomes the sessile polyp which has acquired the power of budding and producing individuals either of its own or of a higher rank; it represents a persistent larval stage and remains in a sexually immature condition as a neutral individual, sex being an attribute only of the final stage in the development, namely the medusa. The polyp of the Leptolinae has reached the limit of its individual development and is incapable of becoming itself a medusa, but only produces medusa-buds; hence a true alternation of generations is produced. ”

“ A living organism, of any kind whatsoever, may be regarded as composed of (1) protoplasm, (2) substances or structures produced by the protoplasm, either by differentiation or modification of the protoplasm itself, or by the excretory or secretory activity of the living substance. The protoplasm of a given organism may be in a single individual mass, or may be aggregated into a number of masses or units, discontinuous but not disconnected, termed cells (see Cytology) . Thus living organisms may be distinguished, in a general way, as unicellular or multicellular. ”

“ The conception of chromatin is one based upon its relations to the vital activities and life cycle, as a whole, of the organism or cell, and not upon any definable material, that is chemical and physical, properties.
The importance of protoplasm, as the physical and material basis of life, has caused it to be the subject in recent years of much minute and laborious research. It seems obvious that matter so peculiarly endowed must possess a complexity of structure and organization far exceeding that which at first sight meets the eye. ”

“ While it cannot be claimed that Bütschli’s theory furnishes in any way a complete explanation of life, leaving untouched, as it does, the fundamental question of assimilation and metabolism, he at least draws attention to a very important class of facts, which, if demonstrated to be of universal occurrence, must be reckoned with in future treatment of the protoplasm question, and would form an indispensable preliminary to all speculations upon the mechanism of the living substance. ”

“ An individual of a given species may contain chlorophyll, with which it decomposes carbonic acid gas in the sunlight, like a plant, while possessing a definite mouth-aperture, by means of which it can ingest solid food, like an animal. Such instances show clearly that in the simplest forms of life the difference between plant and animal is but a difference of habit and of mode of nutrition, to which the organism is not at first irrevocably committed, and which are not at first accompanied by distinctive morphological characteristics. ”

“ The macrogametes, on the other hand, with their great reserves of cytoplasm and nutriment, are more hardy than any other forms of the species, and are able to maintain their existence in periods of famine and starvation when all other forms are killed off. Moreover they may regain the power of multiplication by a process of parthenogenesis, a term originally applied in the Metazoa to cases where a germ-cell of definitely female character, that is to say an ovum, acquires the power of reproduction without fertilization by syngamy. ”

“ The first step towards the formation of a mixed hydroid colony is undoubtedly a hastening of the sexual maturity of the medusa-individual. Normally the medusae are liberated in quite an immature state; they swim away, feed, grow and become adult mature individuals. From the bionomical point of view, the medusa is to be considered as a means of spreading the species, supplementing the deficiencies of the sessile polyp. It may be, however, that increased reproductiveness becomes of greater importance to the species than wide diffusion ”

“ An instance of a unicellular organism is well seen in an Amoeba, or in one of the Foraminifera, classic examples for the study of undifferentiated protoplasm, which here composes the greater part of the body, while products of the formative activity of the protoplasm are seen in the external shell and in various internal granules and structures. As an example of a multicellular organism we may take the human body, built up of an immense number of living cells which produce, singly or in co-operation, a variety of substances and structures, each contributing to the functions of the body. ”

“ By comparing the life-cycles of different forms it is found that syngamy sometimes precedes, sometimes follows, a period of great reproductive activity on the part of the organism. Thus in such a form as Noctiluca, syngamy between two full-grown individuals is followed by rapid sporulation and the production of a swarm of young individuals; on the other hand, in Foraminifera and Radiolaria, rapid sporulation of adult individuals produces a numerous progeny of young forms which may go through the process of syngamy and produce zygotes that simply grow into the adult form. ”

“ In general, it may be said of all these speculations either that they can only be extended to all vital phenomena by the help of so many subordinate hypotheses and assumptions that they become unworkable and unintelligible, or that they only carry the difficulties a step further back, and really explain nothing. Thus it is postulated for Wiesner's hypothetical plasomes that they possess the power of assimilation, growth and reproduction by division; in other words, that they are endowed with just those properties which constitute the unexplained mystery of living matter. ”

“ Life-cycles of the Protozoa.—It is probable that in all Protozoa, as in the Metazoa, the life-history takes its course in a series of recurrent cycles of greater or less extent, a fixed point, as it were, in the cycle being marked by the act of syngamy, or conjugation, which represents, apparently, a process for recuperation of the waning vital powers of the organism. ”

“ The coenosarc constitutes a system by which the digestive cavity of any one polyp is put into communication with that of any other individual either of the trophosome or gonosome. In this manner the food absorbed by one individual contributes to the welfare of the whole colony, and the coenosarc has the function of circulating and distributing nutriment through the colony. ”

“ On the other hand, the morphologist will urge that green plants derive their peculiar powers of metabolism from the possession of very definite cell-organs, namely chromatophores containing chlorophyll; and will argue that living things without such organs must have preceded in evolution those possessing them. The whole dispute is based on the assumption that plant and animal represent the two fundamental modes of metabolism; whereas the study of the Bacteria shows the possibility of many other modes of life. ”

“ But though it may be more scientific, from a theoretical point of view, to group all these primitive organisms together in the way suggested by Haeckel, in practice it is inconvenient, on account of the vast number of forms of life to be comprised as Protista, their diversity in habit of life and organization, and, above all, the difference in the technical methods required for their study, which becomes too complicated for a single worker. Hence Protistology becomes split up in practice by its own mass into three sciences: the Bacteria are the objects of the science of bacteriology ”

“ A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The evolution of the ganglion-cells, is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells ”

“ Sporogony.—This method of reproduction has been described by E. Metchnikoff in Cunina and allied genera. In individuals either of the male or female sex, germ-cells which are quite undifferentiated and neutral in character, become amoeboid, and wander into the endoderm. They divide each into two sister-cells, one of which—the spore—becomes enveloped by the other. The spore-cell multiplies by division, while the enveloping cell is nutrient and protective. The spore cell gives rise to a “spore-larva,” which is set free in the coelenteron and grows into a medusa. ”

“ Returning to the ectoplasm, the excretory function exerted by this layer is seen by the formation in it of the peculiar contractile vacuoles found in most free-living Protozoa. A contractile vacuole is a spherical drop of watery fluid which makes its appearance periodically at some particular spot near the surface of the animal’s body, or, if more than one such vacuole is present, at several definite and constant places. Each vacuole grows to a certain size, and when it has reached the limit of its growth it discharges its contents to the exterior by a sudden and rapid contraction. ”

“ It is difficult to conceive, however, of any single substance, however complex in its chemical constitution, which could perform all the functions of life. To postulate a universal living substance is to proceed along a path which leads inevitably to the assumption of biophores, plastidules or other similar units, since the ultimate living particles must then be imagined as endowed at the outset with many, if not all, of the fundamental properties and characteristic actions of living bodies. ”

“ Apart from larval or embryonic forms there are found typically two types of person, as already stated, the polyp and the medusa, each of which may vary independently of the other, since their environment and life-conditions are usually quite different. Hence both polyp and medusa present characters for classification, and a given species, genus or other taxonomic category may be defined by polyp-characters or medusa-characters or by both combined. If our knowledge of the life-histories of these organisms were perfect, their polymorphism would present no difficulties to classification ”

“ Hence the cavity of the air-sack is equivalent to a sub-umbral cavity in which no manubrium is formed, and the pore or orifice of invagination would represent the margin of the umbrella. In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron. By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore. Thus the disposition of the endoderm-cavities is roughly comparable to the gastrovascular system of a medusa. ”

“ Plus ça change, plus c’est la même chose! Hence true syngamy may be distinguished as karyogamy from the process of plastogamy, or fusion of the protoplasmic bodies, of frequent occurrence in many forms of Protozoa. The individuals whose nuclei undergo fusion are termed gametes. They may be in no way different from each other or from ordinary individuals of the species, or, on the other hand, they may be highly differentiated in size, form and structure. ”

“ Thus the typical hydroid colony starts from a “founder” polyp, which in the vast majority of cases is fixed, but which may be floating, as in Nemopsis, Pelagohydra, &c. The founder-polyp usually produces by budding polyp-individuals, and these in their turn produce other buds. The polyps are all non-sexual individuals whose function is purely nutritive. After a time the polyps, or certain of them, produce by budding medusa-individuals, which sooner or later develop sexual elements ”

“ A very early stage in the degeneration is well exemplified by the so-called “meconidium” of Gonothyraea (fig. 41, A) . Here the medusoid, attached by the centre of its ex-umbral surface, has lost its velum and sub-umbral muscles, its sense organs and mouth, though still retaining rudimentary tentacles. The gonads (g) are produced on the manubrium, which has a hollow endodermal axis, termed the spadix (sp.) , in open communication with the coenosarc of the polyp-colony and serving for the nutrition of the generative cells. ”

“ Protoplasm consists, to the extent of about 60% of its total mass, of a mixture of various nucleo-proteids—that is to say, of those substances which, in molecular structure and chemical composition, are the most complex bodies known. In association with them are always found varying amounts of fats, carbohydrates, and other bodies, and such compounds are always present in the living substance to a greater or less degree as products of both upward and downward metabolism. ”

“ The sensory cells are slender epithelial cells, often with a cilium or stiff protoplasmic process, and should perhaps be regarded as the only ectoderm-cells which retain the primitive ciliation of the larval ectoderm, otherwise lost in all Hydrozoa. The sense-cells form, in the first place, a diffuse system of scattered sensory cells, as in the polyp, developed chiefly on the manubrium, the tentacles and the margin of the umbrella, where they form a sensory ciliated epithelium covering the nerve-centres ”

“ The Hertwigs when they discovered the endoderm-lamella showed on morphological grounds that polyp and medusa are independent types, each produced by modification in different directions of a more primitive type represented in development by the actinula-stage. If a polyp, such as Hydra, be regarded simply as a sessile actinula, we must certainly consider the polyp to be the older type, and it may be pointed out that in the Anthozoa only polyp-individuals occur. ”

“ Thus F. Schaudinn found that in order to observe the sexual union of the gametes of Foraminifera it was necessary to bring together gametes of distinct parentage. On the other hand it has been observed that in many Protozoa, especially in parasitic forms, syngamy takes place between individuals of common parentage. Thus in Amoeba coli, according to F. Schaudinn, a single individual becomes encysted and its nucleus divides into two; after each nucleus has undergone certain maturative changes they give rise to pronuclei which conjugate and initiate a new developmental cycle. ”

“ The archenteron becomes the gastrovascular system or coelenteron. Between the ectoderm and endoderm a gelatinous supporting layer, termed the mesogloea, makes its appearance. The gastrula has now become an actinula, which may be termed the distinctive larva of the Cnidaria, and doubtless represents in a transitory manner the common ancestor of the group. In no case known, however, does the actinula become the adult, sexually mature individual, but always undergoes further modifications, whereby it develops into either a polyp or a medusa. ”