74 6 



GENERATION AND DEVELOPMENT. 



Fig. 211* 



While the dorsal laminae are closing over the primitive 



groove, thickened prolonga- 

 tions of the same serous layer 

 are given off from the lower 

 margin of each of them, and 

 are named lamina viscerales 

 sen ventrales. These visceral 

 laminse by degrees bend 

 downwards and inwards, and 

 at length, enclo'sing a part of 

 the yelk, unite and form the 

 anterior walls of the trunk 



enclosing the abdominal cavity below, as the dorsal plates 



enclose the cerebro-spinal canal above. 



Umbilical Vesicle. 



The ventral laminse, as they extend downwards and in- 

 wards, at first proceed on the same plane with the inner 

 layer of the germinal membrane, which immediately lines 

 them. Soon, however, they show a tendency to turn in- 

 wards, so as to constrict the yelk, and enclose only a part 

 of it ; and soon afterwards the yelk and the inner layer of 

 the germinal membrane that contains it, are separated into 

 two portions, one of which is retained within the body of 

 the embyro, while the other remains outside, and receives 

 the name of the umbilical vesicle (v, fig. 2 1 2). The cavity of 

 the latter communicates for some time with that of the 

 abdomen, through what is called the umbilicus, by means 

 of a gradually narrowing canal, called the vitelline duct; the 

 interior of the abdomen and that of the umbilical vesicle 

 being lined by a continuous layer of the inner stratum, or 

 mucous layer of the germinal membrane ; while around 

 both of them is a continuation of the outer, or serous layer 



* Fig. 211. Diagram showing vascular area in the chick. , Area 

 pellucida. b. Area vasculosa. c. Area vitellina. 



