2S6 



Celt as Basis of Organic Activity 



In the formation of mesoderm, a primitive streak reminiscent of 

 that of the chick embryo develops. The formation of the germ layers 

 in human beings is a very rapid process and all the details are not known. 

 This is, of course, related to the problem of acquiring material of a 

 known age. 



Inner* 



ha^^^ 



Trophoblosf 



■Trophectoderm — '4. 





''***»--*;k:^te^'" 



cavity 



Enfodermal cells 



A Inner Cell Mass Stage 

 (Generalized mammalian) 



D Herlig-Rock 8-day Embryo 



Body 

 stalk 



"v 1 

 C Miller Embryo 



D Refers Embryo 



Fig. 87. — Diagrams indicating schematically the manner in which early human 

 embryos attain their basic structural plan. A, generalized diagram of early mam- 

 malian embryo just prior to implantation. B, Hertig-Rock embryo, 8 days; 

 differentiation within the inner cell mass is just beginning to be apparent, and 

 that part of the ectoderm in contact with the uterine mucosa has started to grow 

 and specialize into flame-shaped trophoblastic extensions. C, Miller embryo, 

 12 days, yolk sac and amnion are becoming clearly recognizable. D, Peters' 

 embryo, 13 to 14 days; the extra-embryonic coelom is lined by mesodermal cells. 

 The names of the embryos refer to the original describers. (From Patten: 

 Human Embryology, The Blakiston Company.) 



