Peter Chalmers Mitchell and Sydney Howard Vines

“ The immediate product of the fusion of cells, or of undifferentiated protoplasm, derived from sexual organs of opposite sex may be generally termed the zygote; but it is not always of the same kind. Thus when two isogametes, or the undifferentiated contents of two gametangia, fuse together, the process is designated conjugation, and the product is usually a single cell termed zygospore. When an oosphere fuses with a male cell, or with the undifferentiated contents of a male organ, the process is fertilization, and the product is a single cell termed oospore. ”

“ REPRODUCTION, in biology, the generation of new organisms from existing organisms more or less similar. It is a special case of growth, and consists of an increase of living substance in such fashion that the new substance is either set free as a new individual, or, whilst remaining attached to the parent organism, separated by some sort of partition so as to have a subordinate individuality. ”

“ In asexual reproduction by spore formation, the spore proliferates without the aid of another spore; in true sexual reproduction the gametes may be regarded as special kinds of spores which appear in two forms, the egg-cell, ovum or female gamete not proceeding to proliferate into a new organism until it has been stimulated by partial or complete fusion with the other form, the spermatozoon or male gamete. The act of fusion or conjugation in question is usually spoken of as fertilization, and the zygote, or starting-point of the new organism, is the fertilized egg-cell. ”

“ It will have been gathered from the foregoing sections that plants generally are capable of both sexual and asexual reproduction; and, further, that in different stages of their life-history they possess the diploid (2x) number of chromosomes in their nuclei, or the haploid (x) number. It may be at once stated that, in all plants in which sexual reproduction and true meiotic spore-formation exist, these two modes of reproduction are restricted to distinct forms of the plant; the sexual form bears only the sexual organs and is haploid; the asexual form only produces spores and is diploid. ”

“ In what may be regarded as the type of normal life-history, the transition from the one generation to the other is marked by definite processes: there is the meiotic development of spores by the sporophyte, and the sexual production of a zygote, or something analogous to it, by the gametophyte. But it has been mentioned in the preceding pages that the transition may, in certain cases, be effected in other ways, which may be regarded as abnormal, though they are constant enough in the plants in which they occur, in fact as manifestations of reproductive degeneration. ”

“ The spermatozoon is one of the smallest of known cells, frequently being no more than one hundred thousandth of the size of the ovum, although the extraordinary case of a small Cypris has been recorded in which the spermatozoa are longer than the animal. It is produced in enormous quantities and relatively to other minute cells is extremely tenacious of life. It may retain its vitality in the male organism for a long time after it has become a separate cell, and may exist for lengthy periods in the female organism. ”

“ Minute investigation has not yet detected any essential structural difference between a sexual cell and a spore; on the contrary, the results so far obtained have established that they essentially agree in being post-meiotic (haploid) . Why then do they differ so fundamentally in their reproductive capacities? Again, sexual cells differ in sex; but there are as yet no facts to demonstrate any essential structural difference between male and female cells. What is known about them tends to prove their structural similarity rather than their difference. ”

“ The various modes in which plants reproduce their species may be conveniently classified into two groups, namely, vegetative propagation and true reproduction, the distinction between them being roughly this, that whereas in the former the production of the new individual may be effected by the most various parts of the body, in the latter it is always effected by means of a specialised reproductive cell. ”

“ It may be mentioned that the stimulating effect of fertilization is not necessarily confined to the female cell; very frequently adjacent tissues are stimulated to growth and structural change. In a Phanerogam, for instance, the whole ovule grows and develops into the seed: the development of endosperm in the embryo-sac is initiated by another nuclear fusion, taking place between the second male nucleus and the endosperm-nucleus: the ovary, too, grows to form the fruit, which may be dry and hard or more or less succulent: the stimulating effect may extend to other parts of the flower ”

“ The female prothallium may consist of many cells with well-developed archegonia, as in the Gymnosperms, or of only a few cells with the female organ reduced to the oosphere, as in the Angiosperms. In either case it is the product of the germination of a megaspore (embryo-sac) which is not shed from its sporangium (ovule) : hence it never becomes an independent plant, and was long regarded as merely a part of the sporophyte until its true nature was ascertained, chiefly by the researches of Hofmeister, who first explained the alternation of generations in plants. ”

“ In the lower (or simpler and possibly degenerate Metazoa) the reproductive or germinal tissue consists of a few cells, sometimes in a group, sometimes scattered and sometimes migratory; in the vast majority of the Metazoa the germinal tissue becomes aggregated in distinct organs, of which those that give rise to ova or female gametes are known as the ovaries, and those that give rise to the spermatozoa or male gametes are known as the testes. The ovary and the testis are the primary reproductive organs ”

“ The question now arises as to the exact seat of the germ-plasm, and the answer is to be looked for in the results of the numerous researches into the structure and development of the reproductive cells that form so large a part of the biological work of recent years. The various facts already mentioned suffice to prove that the nucleus plays the leading part in the reproductive processes of whatever kind: the general conclusion is justified that no reproductive cell can develop into a new organism if deprived of its nucleus. ”

“ The Relation between the Sexual Act and Reproductive Capacity—In the early days of the discovery of the sexual process, it was thought that the capacity for development imparted to the female cell was to be attributed to the doubling of its nuclear substance by the fusion with the male cell. ”

“ In 1664 N. Steno identified the sexual organ of the mammalian female with that of sharks, and first named it the ovary. In 1672 R. De Graaf described the structure of the ovary in birds and mammals, observed the ovum in the oviduct of the rabbit, and repeated Harvey's statement as to the universal occurrence of ova, although he mistook for ova the follicles that now bear his name. ”

“ Clearly sporophytic pangens must be present as well in the spore as in the gametophyte and in its sexual cells. It can only be surmised that they exist there in a latent condition, dominated, as it were, by the gametophytic pangens.
Sexual Reproduction.—Here, again, as yet unanswered questions present themselves. The essence of a sexual cell is that it cannot give rise by itself to a new organism, it is only truly reproductive after the sexual act: this peculiarity is just what constitutes its sexuality. ”

“ In more highly organized plants special organs are differentiated for the production of spores. In the majority of cases the special organ is a sporangium, that is, a capsule in the interior of which the spores are developed; but in many fungi the spores are formed by abstraction from an organ termed a sporophore. In the Thallophyta the sporangium is commonly a single cell. In the Bryophyta it is a multicellular capsule. In the Pteridophyta the sporangium is multicellular, but simple in structure, and this is true also of the Phanerogams. ”

“ The normal sexual act has been described as consisting in the fusion, first, of two cells, then of their nuclei, and finally, often after a long interval, of their chromosomes and of their chromomeres in meiosis. What causes determined these fusions is a question that is only partly answered. It is known in certain cases (e.g. ferns and mosses) that the male cell is attracted to the female by chemical substances secreted for the purpose by the female organ ”

“ There are finally those cases in which apparently vegetative cells take part in the sexual act, as in Phragmidium (Blackman) , where the female organ fuses with an adjacent vegetative cell, and in the fern-prothallium (Farmer) , where the nuclei of two vegetative cells fuse. They would seem to indicate that vegetative cells may, in certain circumstances, contain sufficient germ-plasm to act as sexual organs without being differentiated as such.
An interesting question is that of the origin of apogamy. It is no doubt the outcome of sexual degeneration ”

“ Accessory Reproductive Organs and Processes.—It has been already stated that the primary organs of reproduction in animals are the germinal tissues producing respectively spermatozoa and ova, and that in most cases these are aggregated to form testes and ovaries. In certain animals there are no accessory organs, and when the reproductive products are ripe, they are discharged directly to the exterior if the gonads are external, as in some Coelentera, or if they are internal, break through into some cavity of the body and escape by rupture of the body-wall or through some natural aperture. ”

“ In other cases, as, for instance, Tunicates and many Molluscs, the spermatozoa are discharged, and, being drawn into the body of the female with the inhalent currents, there fertilize the ova. In yet a number of other cases, there is sexual congress without intromittence. The males of many fish, such as salmon, attend the females about to discharge their ova, and afterwards pour the male fluid over the liberated eggs; whilst amongst other fish the males seek out a suitable locality and prepare some kind of nest to which the female is enticed and which receives first the ova and then the milt. ”

“ Secondly, there are the cells which are incapable of independent germination; it is not until these cells have fused together in pairs that a new organism can be developed: these are the sexual reproductive cells or gametes.
In some exceptional cases the normal mode of reproduction, sexual or asexual, does not take place: instead, the new organism is developed vegetatively from the parent. When sexual reproduction is suppressed the case is one of apogamy; when asexual reproduction by spores is suppressed the case is one of apospory. ”

“ Ulothrix, Ectocarpus) where the zoo spores (gonidia) may either germinate independently, or fuse in pairs to form a zygote. Gradually the sexuality of these cells became more pronounced: losing the capacity for independent germination, they acquired the external characters of more or less differentiated sexual cells, and the gametangia producing them developed into male and female sexual organs. But this advancing sexual differentiation did not necessarily deprive the plant of the primitive mode of propagation: the sexual organism still retained the faculty of reproduction by gonidia. ”

“ The nuclear matter that occurs in the tissue cells of animals, when these cells divide, breaks up into a number of chromosomes constant for each kind of animal, and the final stage of cell division is such that each chromosome splits and contributes a half to each daughter cell, so that the latter come to contain the number of chromosomes peculiar to the animal in which they occur. ”

“ Many of the lower invertebrates choose special localities in which to deposit the ova or embryos, and glands, the viscid secretion of which serves to bind the ova together or to attach them to some external object, are frequently present. In many insects, elaborate preparations are made; special food-plants are selected, cocoons are woven, or, by means of the special organ known as the ovipositor, the eggs are inserted in the tissues of a living or dead host, or in other cases a supply of food is prepared and stored with the young larvae. ”

“ True budding is much more common than fission; it occurs in Protozoa, Coelentera, Sponges, Polyzoa, Tunicates and some Flatworms and Chaetopods, the bud being a multicellular portion of the tissues which is partly or completely separated from the parent before it proliferates into the new form. In various larval stages of many animals, asexual reproduction by fission or budding may be produced experimentally or may occur naturally. ”

“ Turning to the Fungi, somewhat similar results have been obtained in the few forms that have been studied from this point of view. In the sexual Ascomycetes it appears (Harper) that meiosis takes place in the ascocarp just before the development of the spores, so that the life-history essentially resembles that of Nemalion. Again, in certain Uredineae, having an aecidium-stage and a teleutospore-stage, which is apparently a sexual process has been observed (Blackman, Christman) which is described in the section on Abnormalities of Reproduction, and the life-history is as follows. ”

“ The protoplasm of all eggs contains nutritive material for the nourishment of the future embryo, and this material may be sufficient in quantity to make the whole cell, although remaining microscopic, conspicuously large, or to expand it to the relatively enormous mass of the yellow yolk of a fowl's egg. ”

“ In many other animals, again; as for instance the frog, the male grasps the ripe female, embracing her firmly for a prolonged period, during which ova and spermatozoa are discharged simultaneously. Where internal fertilization occurs, there are usually special accessory organs. In the female, the terminal portion of the gonad-duct, or of the cloaca, is modified to receive the intromittent organ of the male, or to retain and preserve the seminal fluid. ”

“ The cell nucleus is generally situated near the rounded or pointed extremity, with a centrosome immediately behind it, whilst the scanty protoplasm forms the body and vibratile tail; but there appears to be no general significance in the various configurations that occur amongst different animals. The process of spermatogenesis, or production of spermatozoa from the permanent cells of the testis, varies extremely amongst different animals and has been the subject of many elaborate investigations and much confusing nomenclature. ”