588 



DEVELOPMENT. 



[CHAP. XLI, 



project downwards, and ultimately enclose the anterior portion of 

 the embryo. 



In the anterior dilated extremity of the embryo are developed 

 three vesicles, from which the different parts of the brain are 

 ultimately evolved. 



Formation of the Amnion. The upper layer of the serous 

 lamina becomes raised in the form of a convex ridge, which 

 extends entirely round the circumference of the embryo. This 

 ridge, consisting of course of two layers of serous lamina, rises 

 up gradually, and the two portions from opposite sides approxi- 

 mate above the dorsal surface of the embryo. The parts of 

 the fold corresponding to the anterior and posterior extremities 

 of the embryo grow faster than other portions, and soon two con- 

 siderable reduplications are formed. These are called the cephalic 

 and caudal involucra. The embryo is gradually enclosed by these 

 two layers, which meet over its dorsal surface, and at last coalesce. 

 Thus is formed the amnion; and the interval between the inner 

 layer and the embryo is the amniotic cavity, which contains a fluid, 

 the liquor amnii. The two layers, after some time, become united 

 into one, except at the points where they are separated by the 

 allantois and umbilical vesicle, forming the single amniotic mem- 

 brane, which lies within the chorion. The outer layer, however, 

 is separated from the inner surface of the chorion by a quantity of 

 viscid albuminous matter which is subsequently absorbed. 



The mode of formation of the amnion in mammalian ova is 

 therefore precisely similar to that of the chick, according to 



Fin. 279. 



Fig. 280. 



Plans showing manner of formation of amnion, allantois, and umbilical vesicle, a. Chorion with villi, 

 most abundant in that part beneath which the allantois is seen in figure 280 : this portion ultimately 

 becomes the placenta. 6. Space between the two layers of the amnion. c. Amniotic cavity, d. Situation 

 of intestine, showing its connexion with the umbilical vesicle, e. Umbilical vesicle. /. Situation of 

 heart and vessels, g. Allantois. 





