576 



GENERATION. 



/ty.271. 



[CHAP. XL. 





An ovum also from the upper part of 

 the uterus, showing the further subdivision 

 of the yolk ; but the separate portions are 

 beginning to become incorporated. 



An ovum from the upper part of the 

 uterus, on the sixth day. The yolk-masses 

 have become incorporated into a single 

 mass, which does not entirely occupy the 

 cavity of the zona pellucida. 



All the ova delineated in Fig. 271 ha \cbeen copied from Bischoff 's beautiful Memoir on the develop- 

 ment of the Guinea-pig. Giessen, 185 



It is in this space, or " area germinitiva" that the first traces of the 

 embryo appear ; and it is developed upon the surface of the " ger- 

 minal membrane," or " blastodermic vesicle," being covered only 

 by the zona pellucida. As occurs in the bird's egg, the germinal 

 membrane soon becomes divided into two layers : an external ex 

 " serous layer," or animal lay er } in^v}\.ich thebrain and nervous system 

 and different organs of animal life are developed ; and an internal 

 " mucous" or vegetative layer, from which the alimentary canal and 

 organs of vegetative life take their rise. 



At the same time that these changes are taking place in the 

 interior, the albuminous layer on the external surface of the ovum 

 coalesces with the zona pellucida, forming a single membrane, 

 from which the chorion is formed, or, as maintained by Dr. Barry, 

 the zona pellucida is first removed, and a completely new in- 

 vesting membrane produced. 



Formation of Decidua. Besides the changes which occur in the 

 ovum immediately after conception, which result in the formation 

 of the embryo, and the development of a new covering termed the 

 chorion upon its external surface, alterations of another order are 

 taking place in the uterus, by which its internal surface becomes 

 adapted for providing nourishment for the embryo in its very early 

 stages, before it has reached that period of development when it 

 is enabled to derive its nutriment by direct absorption from the 

 blood of the mother. The change in question consists in the for- 



