Walter Holbrook Gaskell

“ Turning now to the consideration of the second group of tissues, those that are not connected with the central nervous system, we find that they include among them such special cells as the germinal cells, free cells of markedly phagocytic nature, and cells which were originally free and phagocytic, but have settled down to form a supporting framework of connective tissue, and are known as plasma-cells. In the embryo we find also in many cases free cells in the yolk, forming more or less of a layer, which function as phagocytes and prepare the pabulum for the fixed cells of the growing embryo ”

“ It does not seem to me possible to conceive of the gonads arising from cells of the epiblast or of the hypoblast, in the sense that such cells are differentiated cells belonging to a layer with a definite meaning. When we consider that the gonad gives origin to the whole of a new individual, that in the protozoan ancestors of the Metazoa their ultimate aim and object was the formation of gonads, it seems a wrong conception to speak of the gonads as formed from cells belonging either to the gut-wall or to the external epithelium. The gonads must stand in a category by themselves ”

“ If it is permissible to speak of a dorsal and ventral surface to an egg, and we may imagine the egg held with such dorsal surface uppermost, then the yolk would be situated ventrally to the embryo, as in the vertebrate, if the protoplasmic cells of the embryo rose from their central position to the surface through the yolk, while if they sank through the yolk, the yolk would be situated dorsally to the embryo, as in the arthropod. ”

“ Again, it is becoming more and more doubtful whether the peripheral terminations of nerves are ever really free. As far as efferent nerves are concerned the nervous element may entirely predominate over the muscular or glandular, as in the formation of the electric organs of the Torpedo and Malapterurus, but still the final effect is produced by the alteration of the muscle or gland-cell. On the afferent side especially free nerve-terminations are largely recognized, or, as in Barker's book, nerves are spoken of as arising in connective tissue. ”

“ The cells of the germ-band form a small compact area, in which by continual mitosis the cells become more than one-layered, and soon it is found that those cells which lie close against the fluid pabulum form a continuous layer and absorb the nutritious material for themselves and the rest of the embryo. While this area is thus increasing in thickness by continuous development, the group of genital cells remains always apart, increasing in number, but being always in a state of isolation from the cells of the rest of the growing area. ”

“ It is impossible to conceive of an animal possessing a nervous system which is not in connection with sensory and muscular tissues; an isolated nerve-cell is a meaningless possession; but it is equally natural to conceive of a germ-cell being isolated, capable of living an independent existence. ”

“ Just as in a bee-hive the egg-cells may form the fully developed sexual animal, whether drone or queen bee, or the asexual host of workers, so in the body of the Metazoa the free cells may form either male or female germ-cells spermatozoa, or ova, or a host of workers, scavengers, repairers, food-providers, all useful to the community, all showing their common origin by their absolute independence of the nervous system. ”

“ In the adult the free-living lymphocytes and hæmocytes reproduce themselves from already existing free-living cells, but as we pass back to the embryo there comes a time, comparatively late in the history of the embryo, when such free-living cells are not found in the fluids of the body, and they are said to arise from the proliferation and setting free of cells which form a lining epithelium. ”

“ Let us, then, apply this throughout, and see what are the logical results of tracing back the various organs and tissues from the adult to the embryo.
The adult body is built up of different kinds of tissues, which fall naturally, from the standpoint of physiology, into groups. ”

“ As far as the embryo is concerned, there is no great difficulty in the conception that the yolk-mass may have shifted from one side to the other in passing from the arthropod to the vertebrate, for in the arthropod the embryo at first is surrounded by yolk and then passes to the periphery of the egg. ”

“ Such a view of the processes of embryology brings embryology into harmony with comparative anatomy and phylogeny, for it makes the central nervous system and not the alimentary canal the most important factor in the development of the host. ”

“ In the one case the elements are connected together only by what may be called non-living material; there is no direct metabolic activity caused by the action of one cell over a more distant cell in consequence of such connection, it is not a true syncytium; in the second case there is a living connection, the metabolism of one part is directly influenced by the activity of another, and the whole utility of the system depends upon such functional connection. ”

“ I do not think that His' statement by itself would have induced any physiologist to give up the conception of the intimate connection of muscle and nerve, if the work of Golgi, Ramón y Cajal, and others had not brought into prominence the neurone theory, i.e. that each element of the central nervous system is an independent element, without real connection with any other element and capable of influencing other cells by contact only. ”

“ So also, it seems to me, the intimate connection between the metabolism of the gland-cell, as seen in the submaxillary gland, and the integrity of its nervous connection implies that the connection between nerve-cell and gland-cell is of the same order as that between nerve-cell and muscle-cell. Graham Kerr also states in his paper that from the very commencement there is, he believes, continuity between nerve-cell and epithelial cell, but so far he has not obtained sufficiently clear evidence to enable him to speak positively on this point. ”

“ In order to avoid all possibility of our mistaking the clue, the nerve-tube in the embryo always opens into the anus at its posterior end, while in the larval Amphioxus it is actually still open to the exterior at the anterior end. ”

“ In cases where there is no yolk, or very little, as in Lucifer and Amphioxus respectively, the embryo is compelled to feed itself at a very early age; such embryos form a free-swimming pelagic ciliated blastula, the invagination of which, for the purpose of collecting food material out of the open sea, is the simplest method of obtaining nutriment. ”

“ It follows, further, that there must always be continuity of growth in the formation of the host, i.e. in the formation of the neuro-epithelial syncytium; that therefore cells which have been previously free cannot settle down and take part in its formation, as, for instance, in the case of the formation of any part of the gut-epithelium or of muscle-cells from free-living cells. ”

“ Whether the germ-cells appear, as in Hydra, to be derived from the ectoblast, or, as is usually stated, from the endoblast, in neither case ought they to be classed with the internal or external epithelium; they are germ-cells, and the epithelium which they form is neither epiblastic nor hypoblastic, but germinal, forming originally a simple gonocœle, afterwards, in the higher forms, the cœlom with its cells of various function. ”

“ If the mesothelium is composed of cells capable of becoming free, it cannot give rise to the skeletal muscles, and it cannot therefore be right to speak of the skeletal muscles as derived from the lining cells of a part of the primary cœlom. The phylogenetic history of the musculature of the different animals points strongly to its intimate connection with and derivation from surface epithelial cells rather than from cœlomic mesothelial cells. Thus in the cœlenterates, as seen in Hydra, the muscular layer arises directly from a modification of the surface epithelial cells ”

“ A new gut, therefore, in any animal can only be formed from hypoblast, and any theory, such as that advocated in this book, which deals with the formation of a new gut, and does not form that gut from pre-existing hypoblast, must of necessity be wrong and needs no further consideration. ”

“ Again, the process of regeneration after section of a nerve is not in favour of the neuroblast theory. There is no evidence that the cut end of a nerve can grow down and attach itself to a muscular or epithelial element without the assistance of a nerve tube down which to grow. When the cut nerves connected with the periphery degenerate, that applies only to the axis-cylinder and the medullary sheath, not to the neurilemma ”

“ Further, although section of a posterior root in the new-born animal does not affect the development of the nerve-cells in the spinal ganglion, and of the nerve-fibres connecting the posterior root-ganglion with the periphery, it does hinder the development of that part of the posterior root connected with the spinal ganglion. ”

“ In such an animal as Volvox globator we have an indication of the very kind of animal postulated as connecting the single-celled Protozoa and the multi-cellular Metazoa, for it consists of a many-celled case which forms a hollow sphere, each of the cells being provided with flagella for the purpose of locomotion of the sphere, except a certain number which are not flagellated ”

“ Again, with the longer life of the host, the setting free of the gonads no longer necessitating the destruction of the host, and also the gonads themselves requiring a longer and longer time to be fed up to maturity, the bulk and complexity of the whole organism increased and special supporting structures became a necessity. ”

“ In this way I can conceive the formation of a series of both efferent and afferent relays in the nervous system by proliferation of the original neural moiety of the neuro-epithelial elements, every one of which is dependent upon its connection with the peripheral epithelial elements for its due vitality, the whole system being a scheme for co-ordination of a larger and larger number of peripheral elements. ”

“ Naturally enough it cannot pass through all the stages of its past history with equal clearness, for what has taken millions of years to be evolved has to be compressed into an evolution lasting only a few months or weeks, or even less. ”

“ In some Planarians they are limited in number, and of individually large size; in others they are numerous."
When Lankester says that "the lining cells of gonadial chambers are cœlomic epithelium," that is equivalent to saying that the lining cells of the cœlom form an epithelium which was originally gonadial, provided that, as seems to me most probable, his second suggestion, of the cœlom being formed from gonadial mother-cells which have taken up an intermediate position between endoderm and ectoderm and there acquired a vesicular arrangement, is the true one. ”

“ Such cells must be included in the second group, and, indeed, have been said again and again to give origin to the free-living blood-corpuscles of the adult. In other cases they are said to disintegrate after their work is done. ”