1028 EMBRYOLOGY. 



chorion. This sac, which is an expansion of the urachus, is always reversed on one of the 

 sides of tlie amnion ; its two braiicbes are sacculated on their surface like the large intestine 

 ami the greatest forms a cul-de. sac. or conical diverticulum. * 



The extremities of the allantoid coraua appear to have pierced the cliorion ; they form a 

 point covered with a yellow, mucus substance, and separated from the rest of the membrane 

 by a circular constricticu. This part is beneath the chorion, like the remainder of the 

 membrane, only the vessels do not extend beyond the constriction ; so that the elements of 

 the chorion and allantois here undergo a kind of murtiticati(jn. The allantoid infundibulum 

 is encircled by a vascular network that accompanies it throughout tue umbilical cord. 



The epithelium of the allantois is every wliere colourable by iodine reagents, in Kuniiuants. 

 At times the hippomanes is Ibund floating in the liquid it contains. 



Jmntow.— Altogether like that of Solipeds, tiiis membrane is readily resolved into two 



Fig. 564. 



FCETUS OP THE SHEEP, FREED FROM ITS CONNECTION WITH THE UTERUS. 



AL AL, Allantois slightly inflated, seen beneath the chorion ; Am Am Am, amnion slightly dis- 

 tended with fluid underneath the chorioa ; P, P, P, placentae on the surface of the chorion ; C, 

 umbilical chord ; al, al, extremities of the allantoidean cornua, looking as if protruding through 

 the chorion. 



laminae, and presents on its inner surface a great number of little, yellowisii-white, epidermic 

 patches, more especially visible on the amniotic covering of the cord. The epithelium is only 

 stained by iodine at these patches, or villi. These productions are surrounded at their base 

 by a girdle of glycogenic cells. In the foetus of the Cow, at a late stage of gestation, the 

 amniotic fluid is not abundant, and becomes white and viscid ; in one instance we found it 

 stringy, like a solution of gelatine. 



Umbilical cord. — This comprises two arteries and two veins ; the latter forming one trunk 

 on their entering the abdomen. To reach the chorion, tiiese vessels only traverse the amniotic 

 cavity. Tliey are accompanied by tlie urachus, which at their extremity presents the dilatation 

 that results in the allantoid sac. 



Unihilical vesicle. — This pouch disappears at an early period, and not a vestige of it is to be 

 found after the formation of the abdominal parietes. 



