250 



BREEDING 



investing membrane of the yolk (yolk-sac), it will be quite easy to under- 

 stand that at a certain part in the blastodermic membrane a round mass of 

 cells appears, called for the sake of distinction the germinal area. In this 

 round mass, which soon becomes an oval mass, the first sign of the embryo 

 is seen, as shown in the accompanying figure (fig. 538, e). 



On each side the primitive groove or trace above described, are collected 

 two oval masses of cells rising above the plane of the germinal membrane 

 and bending towards each other until they touch and form an arch in which 

 the incipient spinal cord is to be lodged; all this is arranged, it must be 

 observed, in the upper or serous layer of the germinal membrane. Imme- 

 diately below the primitive groove a line of cells may be recognized, forming 

 the chorda dorsalis, the rudimentary stage of the bodies of the bones of 

 the back (dorsal vertebrae). Then below the primitive groove, at the same 



a \ -^ c x^^^^/^y 



i-'ig. 538. The Development of the Ovum 

 a, First division of the ovum ; b, c, d, subdivision of the ovum ; e, first trace of the embryo. 



time that the cells of the laminae dorsalis are closing over to form the 

 central canal for the spinal cord, the serous membrane sends off prolongations 

 from its lower margin, the laminae ventrales, which unite to form the walls 

 of the trunk to enclose the abdominal viscera. 



As they proceed downwards, the ventral laminae turn inwards, enclosing 

 part of the yolk-sac, after which the yolk and inner mucous layer of the 

 germinal membrane are divided into two portions, one being retained in the 

 body of the embryo, the other being left outside. The latter is called 

 the umbilical vesicle. The mucous layer of the germinal membrane now 

 lines the interior of the abdominal cavity and also the interior of the 

 umbilical vesicle. The upper or serous layer is continued round both, and 

 from the portion of the mucous layer enclosed in the body of the embryo 

 the intestinal canal is developed. 



This state of the embryo is represented in the next illustration (fig. 539). 



Foetal Membranes. While the changes above described have been 

 going on, the formation of the fostal membranes, the allantois and amnion, 

 is proceeding. Folds of the external layer of the blastodermic membrane are 

 raised to enclose the body of the embryo forming the amnion ; at the same 

 time during the development of the amnion the allantois protrudes from 



