GENERATION AND DEVELOPMENT 



705 



of cytoplasm and a nucleus, but no centrosome ; the spermatozoon 

 contains no cytoplasm, but both nucleus and centrosome ; the 

 ground common to the two is that they both contain the same 

 number of chromosomes. There is a great deal of recent experi- 

 mental work which tends to show that, if to the ovum a centro- 

 some be supplied, or to the spermatozoon some cytoplasm is 

 given, both may then develop into an ovum without coming 

 together. This remarkable fact has been proved to occur in the 

 eggs of the sea-urchin. 



When segmentation of the ovum is completed, as indicated 

 in Fig. 245 I., a sphere is formed (Fig. 245, IV.), consisting of 



False Amnion or Choric?i 



Villi cf \ 



.\\y/ /ivrta. 

 "v/p Mid gut 



JtNvtcchcrd 

 &*""Ccclcm. 



J Voik Sac. 



Fig. 246. — Diagram of a Transverse Section of a Mammalian Embryo, 

 showing the mode of formation of the amnion (schafer). 



The amnion folds have nearly united in the middle line. 



an outer cellular layer, formed of single cells (Ex. L.), enclosing 

 a cavity known as the segmentation cavity (s.c). The sphere is 

 called the blastodermic vesicle. It contains broken-down yolk, 

 and at one part some smaller cells (I.L.M.), which probably 

 spread over the inner wall of the sphere and form an inner 

 cellular layer. The outer cellular layer is the epiblast ; the inner 

 cellular layer is the hypoblast. While the latter is spreading 

 over the inner wall, a]white disc is developed at one point of the 



45 



