[HO] 



OVUM. 



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 

 different 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 Eutozoa, in 184-1, 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 volk, 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. Mu'ller 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 

 a.s 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 Bcneden, Nord- 

 maun, 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 



