IS THE TKOJ'HOiUiAST OF HVl'OBLASTLU ORIGIN? 589 



Groing' back yet one stage earliei-, in wliich the two first 

 cleavage-cells are just on the point of splitting up into four (as 

 is distinctly indicated by the karyokinetic figures of fig. 30), 

 we notice, besides the two cleavage-cells, three polar bodies 

 of comparative large size, as they are known for mammals in 

 general. Moreover, at the periphery of the two cleavage- 

 cells we find separate nuclei, indicating the very first origin 

 by an early delamination process of the cells which in 

 Text-figs. 2 and 3 constitute the continuous layer of trophoblast. 



Text-fig. 2. 





■■'■Si'j. 



a. '^ 



^ 



=% 



.ji^sm^i. 



W^ 



\ 



^ .*» 



d. 



^''■ 



Fig. 2. 



Galeopitheciis. A series of five sections tbroiio-h a, stage of 

 cleavage preceding the blastocyst of Text-fig. 1. Embryonic 

 knob with bigger nuclei contrasting with the smaller peripheral 

 trophoblast-nuclei. No central space as yet developed. 



In the mammalian genera liitheito e.\auiined with respect to 

 the origin of the trophoblastic layer (Tarsius, Tupaja, rabbit, 

 sheep, pig, stag, dog, mouse, guinea-pig, etc.), the tropho- 

 blast undoubtedly makes irs appearance at a somewhat later 

 stage of cleavage, or rather the distinction in the morida 

 stage between the mother-cells of the embryonic knol) and 

 those of the trophoblast is not so soon evident as it is in 

 Galeopithecus. However, the karyokinetic processes by 

 which in this latter genus the trophoblastic nuclei sepat-ate 

 from the segmentation nucleus (which in its turn owes its 



