THE DEVELOPMENT OF THE EMBRYO 



251 



the hinder portion of the intestinal canal, as a small pear-shaped mass of 

 cells at first, but, rapidly extending, it presses its way between the folds of 

 the amnion and comes in close contact with the outer one of the two folds, 

 becoming more vascular as it proceeds. Reaching the umbilicus, the allan- 

 tois is divided into two parts. The outer part, however, extending to the 

 external investure of the ovum, the chorion, shrivels, and is lost; the other 

 portion remains in the abdominal cavity, and part of it is converted into 

 the urinary bladder, while the 

 remaining portion extends from 

 the bladder to the umbilicus 

 under the name of urachus, 

 which after birth forms one of 

 the ligaments of the bladder. 



It may be remarked here 

 that an oval body flattened in 

 form, which is commonly de- 

 scribed as a false tongue, and 

 sometimes affirmed to exist in 

 the mouth of the foal, is really 

 a concretion which is met with 

 in the fluid of the allantoid 

 sac, and nowhere else; occa- 

 sionally there are several of 

 these bodies, of various sizes. 

 The name given to them, "Hip- 

 pomanes", indicates that they 

 were known to the Greeks, 

 and an ancient superstition 

 attributed to them talismanic 

 power, a belief in which still exists in some parts of the country. 



The annexed figure (fig. 539) shows the arrangement of the three 

 membranes which invest the ovum, i.e. the external chorion, the amnion, 

 the outer portion of which becomes in part firmly attached to the inside 

 of the chorion, and the allantoid sac. 



The villi on the outer surface of the chorion of the human ovum (fig. 540) 

 are seen to be massed on the right side of the figure to form the placenta. 

 In the equine ovum there is no circumscribed placenta, but instead the 

 vascular villi are connected throughout with the internal uterine mem- 

 brane by means of numerous placental tufts, which penetrate the lining of 

 the uterus so that the capillaries of the foetal vessels and those of the 

 maternal vessels are in contact over the whole surface. There is, however, 



Fig. 539. Development of the Embryo, eighteenth day 



, Outer or corneous layer ; b, amnion ; c, allantois con- 

 nected with the anal portion of the alimentary canal ; d, yolk- 

 sac or umbilical vesicle ; e, vitello-intestinal opening; /, simple 

 alimentary canal in lower position; g, trunk and head of 

 embryo; k, foetal heart; i, alimentary canal in upper por- 

 tion; k, place of convergence of amnion and reflexion of 

 false amnion or corneous layer. 



