I 111! HKT.M. MliMlil: M \N. 



251 



the s.-fi-.-t ion i.f ainniotir water can become much greater, and < 



by a con>iderablr distension of tin- aiiinioii. It-ad to conditions which 

 have been called dropsy of the amnion, or liydnminimi. 



3. The Yolk Sac. 



Tin- i/oJk-sac or the uml'itiritl mick (vesiculu ninl.ilicalis) in Man 

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



i n e re a s i n 



ainnion, 



shrivels 



and 

 to a 

 structure that 

 easily escapes 



r vat ion. 

 In human 

 foetuses of the 

 second a n d 

 third week (fig. 

 144) the yolk- 

 sac (ds) fills 

 somewhat more 

 than half of the 

 1 'lasted er mic 

 vesicle and is 

 not constricted 

 off from the in- 

 testine, which 



still has the 

 form of a 



Sch 



47 

 <**__ 



\ 



Tig. 144.-Human embryo with yolk-sac, amnion, and blly-talk of 

 15 to 18 days, after Co M^L-lilii-ln.- Kinl-ryoncn "). 



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

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

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

 COSTF.'S fig. 4. The chorion is detached at a >u'. ant, Amnion ; on 1 , 

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

 a tip; btt, belly-stalk; Sch, tail-end; us, primitive segment; dg, 

 vitelline blood-vessels; dt, yolk-wic ; h. In-art; c>>. visceral arch. 



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

 a thick tttiill,- or citrUim' ,lnct with the middle of the rudimentary 

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

 the vasa 01 uphalomesent erica. 



During the sixth week the vitelline duct or ductus oinphalome-en 

 tericus has grown out into a long, narrow tuhe, wliich sooner or later 

 Ltfl cavity and is converted into a solid epithelial cord. It 

 terminates in the small egg-shaped umbilical vesicle (n'gs. 139 D and 

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

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

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



