8oo 



EMBRYOLOGY 



process of its formation is called gastrulation. It is at this portion of 

 the ovum that the embryonic spot, or area, afterward appears. 



The albuminous covering which the ovum has received in the upper 

 part of the Fallopian tube gradually liquefies and penetrates the vitel- 

 line membrane, furnishing, it is thought, matter for the nourishment and 

 development of the ovum. In the Fallopian tube, indeed, the adventitious 

 albuminous covering presents an analogy to the albuminous coverings 

 which the eggs of oviparous animals receive in the oviducts ; with the 

 difference that this albuminous matter is almost the sole source of 

 nourishment in the latter and exists in large quantity, while in viviparous 

 animals the quantity is small, is usually consumed as the ovum passes 



into the uterus, and in the uterus 

 the ovum forms attachments to 

 and draws its nourishment from 

 the vascular system of the mother. 

 Primitive Streak. Soon after 

 the formation of the blastula, at a 

 certain point on its surface there 

 appears a rounded elevation or 

 heap of smaller cells, forming a 

 distinct spot, called the embryonic 

 spot. As development advances, 

 this spot becomes elongated and 

 oval and is called the embryonic 

 shield. It is then surrounded by 

 a clear oval area, called the area 

 pellucida, and the area pellucida 

 is itself surrounded by a zone of 

 cells, more granular and darker 

 than the rest of the blastoderm, 

 called the area opaca. The line thus formed and surrounded by the area 

 pellucida is called the primitive streak or trace. This primitive streak, 

 or groove, however, is a temporary structure. After the groove is 

 formed, there appears, in front of but not continuous with it, a new fold 

 and a groove leading from it. This is the " head-fold," and the groove 

 is the true medullary groove, which afterward is developed into the 

 neural canal. 



Blastodermic Layers. The blastodermic cells, resulting originally 

 from the segmentation of the vitellus, are first split apparently into two 

 layers, the external, or epiblast, and the internal, or hypoblast. The 

 epiblast is developed into the epidermis and its appendages ; the glands 

 of the skin ; the enamel of the teeth ; the mucous lining of the mouth, 



Fig. 222. Embryonic shield of a rabbit, show- 

 ing the primitive streak and the medullary groove 

 above, x 28 (Kollmann). 



