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- 

 i n c re asin g 

 amnion, 

 shrivels 



am 1 



bst 



Sch 



dg 



dt 



and 

 to a 



structure that 



easily escapes 



observation. 

 In human 



foetuses of the 



second and 



third week (fig. 



144) the yolk- 



Fac (da) fills 



somewhat more 



than half of the 



lilastodermic 



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 stalk 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 D 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), 



Fig. 144. Human embryo 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 lef b side being represented in 

 COSTE'S fig. 4. The chorion is detached at am 1 , am, Amnion ; am 1 , 

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

 a tip ; bst, belly-stalk ; Sch, tail-end ; iis, primitive segment ; dg, 

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



