76 



THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



Amnion (cut) 

 ^^- Plural fold 



Yolk sac 



cavity, and hence called the neurenteric canal (p. ii). The fore-gut and head 

 fold have formed at this stage and there are branched chorionic vilH. Somewhat 



more advanced conditions are found in 

 an embryo 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 appendages (Fig. 79). 

 The fore- and hind-gut are well de- 

 veloped, the amniotic cavity is large, 

 and the yolk sac still communicates 

 with the gut through a wide opening. 

 The allantois is present as a long curved tube somewhat dilated near its 

 blind end and embedded in the mesoderm of the body stalk. As the hind-gut 



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- 

 Heart' 



Body stalk 

 Allantoic 

 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. 



develops, the allantois comes to open into its ventral wall. A large umbilical 

 artery and vein are present in the body stalk. 



In an embryo of 23 somites 2.5 mm. long, described by Thompson, the allan- 



