EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



77 



tois has elongated and shows three irregular dilatations (Fig. 80). 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 eventuaUy atro- 

 phies. That part of the aUantois extending from the umbilicus to the cloaca of the 

 hind-gut possibly takes part in forming the bladder and the urachus, a rudiment 

 extending as a soHd cord from the fundus of the bladder to the umbihcus. 



The human allantois is thus small and rudimentary as compared with that 

 of birds and Ungulates. As we have seen, the cavity is very large in the pig, and 



Haller found an allantoic sac two feet 

 x'""^ \ long connected with a goat embryo of 



two inches. In human embryos it ap- 



/ 



Pharynx— -/^.^ 

 Pharyngeal _^ 

 mevibi'unc 

 Thwcoid 



Pericardium 



Hepatic dircrliciilum 

 Septum Iransvcrsum 



Yolk stall 



Allantois 



Cloacal membrane 

 Cloaca 



Neural folds 



Neurenteric canal 



Fig. 80. — Median sagittal section of a 2.5 

 mm. human embryo showing digestive tract 

 (after Thompson). X 40. 



Fig. 81. — Human embryo of 2.11 mm. (Eternod). 

 X 35. 



pears very early and is not free, but embedded in the body stalk. Its func- 

 tions, so important in birds and Ungulates, are in man performed by the chorion. 

 Yolk Sac and Yolk Stalk. — In the youngest human embryos described, the 

 entoderm forms a somewhat elongated vesicle (Fig. 76). With the development 

 of the fore-gut and hind-gut in embryos of 1.54 and 2 mm. (Figs. 77 and 78), the 

 entodermal vesicle is di\'ided into the dorsal intestine and ventral yolk sac, the two 

 being connected by a somewhat narrower region. This condition persists in 

 an embryo 2.5 mm. long (Fig. 80). In the figure most of the yolk sac has been 



