68 CLEAVING OF THE YOLK 



multiplication of the cells. For, it is not until two cells have appeared 

 in place of the one which previously occupied the mass of the yolk, that 

 the latter begins to undergo division. Kb'lliker therefore regards the divi- 

 sion and subdivision of the yolk, as the consequence of an attractive force 

 exerted by the germ-cells on the vitelline or yolk-substance.* And Dr. 

 Sharpey ] remarks, that the shrinking of the granular mass of the yolk 

 around the first central cell, is in harmony with this view. Dr. Sharpey 

 also mentions the fact, that on one occasion while he was examining the 

 ova of one of these Ascarides, at the time when one of the large segments 

 into which the yolk is first cleft, divided itself into two portions, he 

 observed a very obvious heaving motion among the granules throughout 

 the whole mass; then ensued a constriction at the circumference, which, 

 proceeding inwards, soon completed the division.^ 



Kolliker thinks that the earlier divisions of the yolk cannot be re- 

 garded as cells, for they appear to him not to be enveloped with a mem- 

 brane. Dr. Sharpey, on the other hand, is disposed from his observations 

 on the ova of Ascaris, to admit the existence of an enveloping membrane, 

 and consequently he regards these larger, as well as the later and smaller 

 subdivisions of the yolk, as complex cells, analogous in their structure to 

 the unimpregnated ovum, and to the nucleated globules of the nervous 

 system. || 



* L. c. p. 108. t Loc. cit. 



J In relation to this question concerning the cause of the division of the yolk, may be also 

 mentioned the results of some observations recently made by Vogt on the development of the 

 Molluscous Gasteropods. (Ann. des Sci. Nat. 1846. Zoologie, p. 23, et seq.) According 

 to this embryologist, the yolk of the ovum of Acteon viridis (on which his observations were 

 almost exclusively made) consists, immediately after impregnation, of a gelatinous substance 

 containing numerous minute granules, and having in its centre a round transparent vesicle 

 appearing as a clear space. Shortly afterwards the vitellary mass divides into two equal 

 portions, in the centre of each of which is contained a clear vesicle, like that before observed 

 in the centre of the yolk itself. But, contrary to the above-stated opinion of Kolliker, 

 and to that of most other embryologists, Vogt believes that, at least in Acteon, the 

 division of the vitellary mass, instead of being a consequence of the multiplication of the 

 central vesicle, precedes, and is the cause of, the latter phenomenon. The only evidence, 

 however, on which this opinion appears to be based is, that in one instance Vogt ob- 

 served an ovum in which one of the two portions into which the yolk was dividing was 

 somewhat smaller than the other, as if of more recent formation, and did not contain a 

 vesicle. 



Quain's Anatomy, p. 1. 



|| According to Vogt's observations, the very earliest divisions of the yolk-mass, in the 

 ovum of Acteon, are unprovided with an enveloping membrane, yet, by the time the 

 number of segments has amounted to twenty-four, evidences of an investing membrane 

 around each may usually be observed. (Further observations on this subject will be made 

 when considering the process of division and subdivision of the yolk as it occurs in the 

 mammiferous ovum.) A remarkable peculiarity has also been observed by Vogt in the pro- 

 ducts of the division of the yolk in this animal. The first two divisions of the vitellary mass 

 subdivide as in the ova of other animals, and produce four equal-sized spheres, arranged 

 together in a crucial form, and each possessed of a central transparent vesicle. But in the 



