THE FCETAL MEMBRANES OF MAN. 



251 



the secretion of amniotic water can become much greater, and can, 

 by a considerable distension of the amnion, lead to conditions which 

 have been called dropsy of the amnion, or hydramnion. 



3. The Yolk-Sac. 



The yolk-sac or the umbilical vesicle (vesicula umbilicalis) in Man 

 pursues the opposite course of development from that of the ever- 

 increasing 

 amnion, and 

 shrivels to a 

 structure that 

 easily escapes 

 observation. 



In human 

 foetuses of the 

 second and 

 third week (fig. 

 144) the yolk- 

 ?ac (ds) fiUs 

 somewhat more 

 thanhalf of the 

 blastod e r m i c 

 vesicle and is 

 not constricted 

 off from the in- 

 testine, which 

 still has the 

 form of a 

 groove. 



In somewhat older embryos it is seen to be connected by means of 

 a thick sialic or vitelline duct with the middle of the rudimentary 

 intestine, now converted into a tube. It is supplied with blood by 

 the vasa omphalomesenterica. 



During the sixth week the vitelline duct or ductus omphalomesen- 

 tericus has grown out into a long, narrow tube, which sooner or later 

 loses its cavity and is converted into a solid epithelial cord. It 

 terminates in the small egg-shaped umbilical vesicle (figs. 139 Z> and 

 143 nb). Since the amnion, in consequence of a greater accumulation 

 of fluid, now fills the whole blastodermic vesicle (fig. 143), it has 

 enveloped both the vitelline duct and the neck of the allantois (al), 



Tig. 144. — Human em1)ryD with yolk-sac, amnion, and belly-stalk of 

 15 to 18 days, after CosTE, from His (" Menschliche Embryonen "). 



His has untwisted somewhat the posterior end of the body in com- 

 parison with the original figure, in order to bring into view the 

 right side of the end of the body, the left side being represented in 

 Coste's fig. 4. The chorion is detached at am*, am, Amnion ; am\ 

 the point of attachment of the amnion to the chorion drawn out to 

 a tip ; bst, belly-stalk ; Sell, tail-end ; us, primitive segment ; dg, 

 vitelline blood-vessels ; ds, yolk-sac ; h, heart ; vb, visceral arch. 



