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 



Mesoderm 

 of yolk sac 



Blood vessel 



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



Mammals, but the extent of the extraembryonic mesoderm, at 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 Spec. 



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 



