92 



TEXT-BOOK OF EMBRYOLOGY. 



mesoderm. In Graf Spec's ovum both longitudinal and transverse sections 

 would suggest the same origin for the intraembryonic mesoderm as in lower 



Chorionic villi 



- Blood vessel 



FIG. 85. Medial section of human embryo shown in Fig. 84. von Spee, Kollmann. 



Mammals, but the extent of the extraembryonic mesoderm, at this early stage 

 oi the embryonic disk, would indicate a departure from the conditions seen in 

 the hwer Mammals. In other words, it scarcely seems possible that the 



Ecto- 

 Mesoderm derm Primitive groove 



Ectoderm_ 



Parietal mesoderm - 



Visceral mesoderm 



Entoderm- 



& 



FIG. 86. Transverse section through primitive streak of embryo shown in Fig. 84. von Spee. 



mesoderm which lines the chorionic vesicle and covers the yolk sac has grown 

 out from the mesoderm which arises within the embryonic disk; it seems more 



