90 TEXT-BOOK OF EMBRYOLOGY'. 



young ovum described by Peters shows all three germ layers and the flat embry- 

 onic disk; but there are no stages between the two to show how these structures 

 develop. 



A section through the ovum described by Peters (Fig. 82) shows the ectoderm 

 as a flat layer of stratified or pseudostratified cells, the margin of which is re- 

 flected dorsally as the lining of the roof of the amniotic cavity (compare Fig. 5 2, c) . 

 Beneath the ectoderm is a layer of cells the mesoderm which is continu- 



Coagulum Trophoderm Uterine epithelium 





Embryonic 

 Yolk sac . ! Amni n .>f-"*$ 



>y' 



A, * 



'* 



i / ;> . 



Blood 



FIG. 83. Section through very young human chorionic vesicle embedded in the 



uterine mucosa. Peters. 



The vesicle measured 2.4 x 1.8 mm., the embryo .19 mm. Peters reckoned the age as 3 or 4 days, 

 but later studies of other embryos go to show that the age is much greater; Bryce and 

 Teacher estimate it at 14 to 15 days. 



ous at its margin with the mesoderm of the roof of the amnion, with mesoderm 

 lining the chorionic vesicle, and also with the mesoderm covering the yolk sac 

 (Fig. 83) Beneath the mesoderm of the embryonic disk is a layer of entoderm 

 which also extends ventrally to line the yolk sac. There is here no trace of an 

 invaginated entoderm from which the mesoderm might arise. 



Graf Spec has described an ovum somewhat older than Peters', in which the 

 embryonic disk shows certain features which are comparable with those in 

 lower Mammals. On surface view (Fig. 84), the primitive groove is especially 



