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Credit: Ovum / by Allen Thomson. Source: Wellcome Collection.
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![[140] their appearance. According to most ovo- logists, the last globules formed by segmenta- tion are the nucleated organised cells im- mediately constituting the blastoderma. A dilFerent view of the process, however, in Mammalia, has been taken by BischofF, very decidedly set forth in his two most recent works on the development of the guinea-pig and the deer; according to which the last resulting spherules formed by segmentation are not true cells, and that previous to the formation of the blastodermic cells, the yolk- germ falls completely into an amorphous or homogeneous finely granular substance, out of which, secondarily, the blastodermic cells are produced by a process of cytogenesis. It seems probable that, in the different classes of animals, there may be considerable variety in the degree of perfection in organisation or ad- vance in cell-structure to which the segments of the yolk have attained at the period when the development of the embryo begins to ma- nifest itself. But in the higher animals at least the weight of evidence appears to me in favour of the view that the process of segmentation results directly in the formation of blastodermic cells. The fact now established by the obser- vations of Reichert in Entozoa, in 1841, of Ransom in osseous fishes, and more particu- larly those of Remak in Batrachia, that a de- licate membrane is formed over the surface of each of the segments as they appear, and that the last and smallest segments possess a deli- cate membranous envelope, appear to show that, in these animals, each segment has the structure of an organised cell, and is very si- milar to, if not identical with, those of the blastodermic lamina. The origin of the embryo-cell is still in- volved in obscurity. Most ovologists are dis- posed to connect it in some way or other with the previously existing germinal vesicle, or some part of its contents, and more especially the nucleus. For the solution of this ques- tion, as already remarked, a more accurate knowledge of what happens to the germinal vesicle at the time of that disappearance which has been so commonly observed at the period of the maturation of the ova of almost all ani- mals, will be required. Does the macula re- main, as has been alleged by some, to form the nucleus or the whole of the embryo-cell? Or, in other cases, if the multiplied maculae are dispersed among the granules of the ger- minal yolk, are they again collected together into a mass or spherule to form the embryo- cell? Or, again is the embryo-cell formed out of other materials, and not necessarily either partially connected with, or wholly de- rived from, the germinal vesicle ? And finally, might it not be, according to some recent ob- servations, such as those of J. Muller on En- tochoncha and those of Remak on the frog, that the disappearance of the germinal vesicle is not attended with the dispersion of its con- tents, but is a phenomenon caused only in a certain number of animals by the solution of the delicate external wall of the vesicle, and by some change in the position and consist- ence of its contents? Further observations will be required to determine this point; but if in the meantime we regard it as most pro- bable that the embryo-cell is in some way or other connected in its origin with the germinal vesicle, we might further found upon this the speculative view that the blastodermic cells and the blastema from which unques- tionably, by a histogenetic process of cell-di- vision and multiplication, the various textures and organs of the animal body are produced, may be regarded as the descendants of the original cell-germ from which the ovum was developed combined with the sperm. We should thus trace the organic cellular connec- tion between the succession of parents and offspring, which I have stated to be one of the most general facts in organised nature. The observations respecting the very re- markable movements of the yolk, before and during the earlier stages of the segmenting process which have now been recorded by several physiologists, must excite the liveliest interest, and suggest subject for much reflection as to the evidence they may afford of the causes of this change, or, if we may use the expression, of the forces by which segmenta- tion is brought about. There seems to be little doubt that the embryo-cell (and its nu- cleus first of all) is the earliest to become di- vided, and that the process of cleavage then proceeds from the surface of the segmenting mass inwards towards the cell ; but in what relation the nucleus, granular substance of the yolk, and ovicell-membrane stand to each other in this process, must be left to be de- termined by future researches. Of the other early changes in the ovum which immediately follow fecundation and precede embryonic development little need here be said. They consist principally in the greater degree of consolidation and compact- ness acquired by the germinal part of the yolk, and in the formation in most animals of a clear space between the surface of the yolk- substance and the enclosing vitelline mem- brane. It is in this clear space, or, as it has been called by Newport, respiratory chamber, that the spermatozoa have been observed in those instances in which they have been as- certained to penetrate into the cavity of the ovum. There is another phenomenon of the same period, which has now been so frequently observed, and which is of so peculiar a nature, that it must not be passed over without no- tice ; I allude to the appearance in the re- spiratory space of one or more clear and highly refracting spherules, nearly of the size of the germinal vesicle, but quite independent of it. These clear hyaline-like globules have been observed in the ova of Gasteropodous Mol- lusca after fecundation by almost all those who have attended to the ovology of this class of animals, among whom may be mentioned Dumortier, Pouchet, Van Beneden, Nord- mann, and Vogt; in the Annelida by Quatre- fages ; in Mammalia by Bischoff and Barry; and in Batrachia by Newport. From the observations of Quatrefages in Hermella they appear to be excluded or expressed, as it were, from the clear basement-substance of](https://iiif.wellcomecollection.org/image/b24918751_0146.jp2/full/800%2C/0/default.jpg)
