PROCESS OF DEVELOPMENT IN MAMMALIAN OVA. .533 



still but little expanded germ lies over the cavity, the larger 

 cells lying on the floor of the cavity be followed towards the 

 periphery, it may be seen that they are directly continuous 

 with the deep layer of large cells which forms the lower layer of 

 the thickened border, and, as it would further appear, the rudi- 

 ment both of the motorial and intestinal glandular layer. 



This relation renders it highly probable that the large cells 

 on the floor of the germ cavity migrate towards the periphery, 

 in order to form or to strengthen the large-celled deposit there 

 found. It is further to be remarked that a rich plexus of blood- 

 vessels develops beneath the walls of the yolk sac, and that 

 the large cells upon the floor of the cavity may fulfil some pur- 

 pose in the development of this plexus. 



Fig. 405. 



Fig. 405. Section of the germ of Salmo fario. 



(d.) MAMMALS. In regard to the first changes occurring in the 

 germ of Mammals, I have scarcely myself made any observa- 

 tions worthy of notice. In this domain I am therefore only a 

 compiler. And, however valuable the literary material before 

 me may be, I am still unable to make more than a limited use 

 of it ; first, because I am unable to give here a history of the 

 controversies by which the first positive facts were established ; 

 and secondly, because the Mammalian ovum has scarcely been 

 worked over during the most recent epoch of embryology. 



The older statements do not accord well with modern modes 

 of expression, and I have no inclination to reconcile the differ- 

 ences at the desk. I prefer to give a concise account, and at 

 the same time to call attention to the fact that in the history 

 of the development of the Mammalian ovum there is a rich 

 field of inquiry to be worked over. 



Bischoff first gave an accurate account of the cleavage of the 



