7 8 



HUMAN EMBRYOS AND FETAL MEMBRANES 



Amnion (cut) 

 , Neural fold 



Yolk sac 



cavity, and hence called the neur enteric canal (p. 34). The fore-gut and 

 head fold have formed at this stage and there are branched chorionic 



villi. Somewhat more advanced 

 conditions are found in an em- 

 bryo of 1.8 mm. with five to six 

 pairs of segments (Fig. 78). 



A reconstruction by Dandy 

 of Mall's embryo, about 2 mm. 

 long with seven pairs of segments, 

 shows well the embryonic ap- 

 pendages (Fig. 79). The fore- 

 and hind-gut are well developed, 

 the amniotic cavity is large, and 

 the yolk sac still communicates 

 with the gut through a wide 

 opening. The allantois is present 



as a curved tube, somewhat dilated near its blind end and embedded in 

 the mesoderm of the body stalk. As the hind-gut develops, the allantois 



Chorion 



Amnion 



Neurenteric canal 

 Primitive streak 



Body stalk 

 Chorion 



FIG. 78. Kromer human embryo of 1.8 

 mm., in dorsal view (after Keibel and Elze). 



X 20. 



Pharyngeal 

 membrane 

 Fore-gut 

 Heart 



Body stalk 

 Allanloic 



stalk 

 Hind-gut 



Splanchnic 

 mesoderm 



Blood island 

 Blood vessel 



FIG. 79. A human embryo of 2 mm. in median sagittal section (adapted from reconstructions 

 of Mall's embryo by F. T. Lewis and Dandy). X 23. 



comes to open into its ventral wall. A large umbilical artery and vein 

 are present in the body stalk. 



