SUBSEQUENT TO CLEAVING. 81 



instead of its late mulberry-like appearance. The ovum indeed ap- 

 pears at first sight to have lost all trace of the cleaving process, and, 

 with the exception of being paler and more translucent, almost exactly 

 resembles the ovarian ovum ; its yolk consisting, apparently, of a confused 

 mass of finely granular substance.* But on more careful examination it 

 is found that these granules are aggregated into numerous minute 

 spherical masses, each of which contains a clear vesicle in its centre, but 

 is not, at this period, provided with an enveloping membrane, and pos- 

 sesses none of the other characters of a cell.f The zona pellucida, and 

 (in the rabbit) the layer of albuminous matter surrounding it, have at this 

 time the same characters as when at the lower part of the Fallopian tube. 



Shortly after this, important changes begin to ensue. Each of the 

 several globular segments of the yolk becomes surrounded by a membrane, 

 and is thus converted into a cell, the nucleus of which is formed by the 

 central vesicle, the contents by the granular matter originally composing 

 the globule ; these granules usually arrange themselves concentrically 

 around the nucleus.J When the peripheral cells, which are formed first, 

 are fully developed, they arrange themselves at the surface of the yolk 

 into a kind of membrane, and at the same time assume a pentagonal or 

 hexagonal shape from mutual pressure, so as to resemble pavement 

 epithelium. As the globular masses of the interior are gradually con- 

 verted into cells, they also pass to the surface and accumulate there, thus 

 increasing the thickness of the membrane already formed by the more 

 superficial layer of cells, while the central part of the yolk remains filled 

 only with a clear fluid. By this means the yolk is shortly converted into 

 a kind of secondary vesicle, situated within the zona pellucida, and named 

 by Bischoff, "vesicula blastodermica." The similarity of the several parts 

 of this process with those observed by Vogt to take place in the ova of 

 the frog (page 77) is very striking, and it is probable that the series of 

 changes in the one are exactly analogous to those in the other. While 

 these changes are proceeding within the yolk, which is at the same time 

 gradually increasing in size, the zona pellucida and the layer of albuminous 

 matter outside gradually coalesce, and so form a single membrane, the 

 external investment of the ovum, from which the chorion is shortly 

 afterwards produced. 



In consequence of these changes, the Mammiferous ovum, when exa- 

 mined at this period, is found to consist of two nearly transparent vesicles 

 enclosed one within the other, but differing from each other in composi- 

 tion. || The external vesicle, which in the rabbit is formed, as above 

 noticed, by the coalescence of the zona pellucida and albuminous covering, 



* Bischoff, Entwickelungs-geschichte des Kaninchen-eies, p. 85. t Op. cit. p. 86. 

 Bischoff, op. cit. p. 92, and Entwick. des Hunde-eies, p. 65. Op. cit. p. 90. 

 || See the account given by Von Baer, Wagner, and others, who also describe the two 

 coats of the ovum. Muller's Physiology, p. 1561. 



G 



