86 



THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



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 long connected with a goat embryo of two 

 inches. In human embryos it appears very early and is not free but embedded 



in the body-stalk. Its functions, so 

 important in birds and Ungulates, are 

 in man performed by the chorion. 



Neural folds 



Neurenteric canal 



Fig. 



-Median sagittal section of a 2.5 Fig. 76.— Human embryo of 2. 11 mm. (Eternod). 



mm. human embryo showing digestive tract 

 (after Thompson). X 40. All., allantois; 

 CI., cloaca; C. per., pericardial cavity; Div.hep., 

 hepatic diverticulum; D. v., ductus vitellinus 

 (yolk-stalk); gl. th., thyreoid gland; Men. cl., 

 cloacal membrane; PA., pharynx; Sept. tr., sep- 

 tum transversum. 



Yolk-Sac and Yolk-Stalk.— In the 



youngest human embryos described 

 (Peters) the entoderm forms a some- 

 what elongated vesicle. With the 

 development of the fore-gut and hind- 

 gut in embryos of 1.54 and 2 mm. (Figs. 73 and 74), the entodermal 

 vesicle is divided into the dorsal intestine and ventral yolk-sac, the two 

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

 an embryo of 2.5 mm. long (Fig. 75). In the figure most of the yolk-sac 

 has been cut away. An embryo with 9 pairs of segments, with three brain 

 vesicles and with the amnion cut away is seen in Fig. 76. The relation 

 of the fetal appendages to the embryo shows well in the embryo of Coste (Fig. 



