92 



TEXT-BOOK OF EMBRYOLOGY. 



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

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



-Cho 



. Mesoderm 

 of chorion 



Belly stalk 

 Primitive streak 



Blood vessel 



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



Mammals, but the extent of the extraembryonic mesoderm, a? this early stage 

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

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



Ecto- 

 Mesoderm derm Primitive groove* 



Ectoderrr 



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 



