EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



85 



Body-stalk 



fore the development of the fore-gut or hind-gut. In Peter's embryo the amnion, 

 chorion and yolk-sac are present but not the allantois (Fig. 72). In an embryo 

 1.54 mm. long, described by Von Spee (Fig. 73 A, B), there is no hind-gut, but the 

 allantoic diverticulum of the entoderm has invaded the mesoderm of the body- 

 stalk. This embryo, seen from the dorsal side with the amnion cut away, shows 

 a marked neural groove and primitive streak. In front of the primitive knot a 

 pore is figured leading from the neural groove into the primitive intestinal 

 cavity, hence called the neurenteric canal. The fore-gut and head-fold have 

 formed at this stage and there 

 are branched chorionic villi. 



A reconstruction by Dandy 

 of Mall's embryo, about 2 mm. 

 long with seven pairs of seg- 

 ments, shows well the embryonic 

 appendages (Fig. 74). 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 open- 

 ing. The allantois is present as a 

 long curved tube somewhat di- 

 lated near its blind end and em- 

 bedded in the mesoderm of the 

 body-stalk. As the hind-gut de- 

 velops, 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 allantois has elongated and shows 

 three irregular dilatations (Fig. 75). A large cavity never appears distally in 

 the human allantois as in Ungulates. When it becomes included in the umbilical 

 cord its distal portion is tubular and it eventually atrophies. That part of the 

 allantois extending from the umbilicus to the cloaca of the hind-gut takes part 

 in forming the urogenital sinus, the bladder and the urachus, a rudiment extend- 

 ing as a solid cord from the fundus of the bladder to the umbilicus. According 

 to Felix, the allantois forms only the urachus and a portion of the bladder. 



mc 

 mesoderm 



vessel 



Fig. 74. — A human embryo of 2 mm. in median 

 sagittal section (adapted from reconstructions of Mall's 

 embryo by F. T. Lewis and Dandy). 



