566 



THE EMBRYOLOGY OF THE FROG 



enteron and neural tube. This opening is called the neurenteric canal. 

 (Fig. 328.) 



Just as with the chick, the confluence of the two lateral walls of 

 the blastopore (which have been formed from the remains of the germ- 

 ring) bring a median cell mass together in the axial region, in which 

 ectoderm, entoderm, and mesoderm, are fused in a quite undifferentiated 

 mass. This is the primitive streak. The groove which lies in the mid- 

 line of the primitive streak is called the primitive groove. 



It is from this primitive streak that ectodermal cells are budded 

 forth into the neural folds and upon the surface of the body. It is from 

 the primitive streak, also, that mesodermal cells are budded off into the 

 lateral bands, and entodermal cells into the walls of the archenteron. 



At this time the chief characteristic of the brain is the single flexure 

 around the tip of the notochord (Fig. 329). The hypophysis (pituitary 

 body) can be seen as a tongue of ectodermal cells just beneath the end 

 of the fore-brain ; it extends inward a short distance. 



The rudiments of the eyes (Fig. 326, au) are indicated as small 

 patches of the deeply pigmented ectodermal epithelium in an antero- 

 lateral region of the medullary plate. 



The rudiments of the ears (Fig. 282, C) are seen as thickened 

 patches of the inner or nervous layer of ectoderm opposite the region 

 of the hind-brain. They are difficult to see externally as yet. 



The rudiments of the olfactory organs are formed as thickened 

 patches of ectoderm below and in front of the optic rudiments. The tiny 

 depressions on the surface which are to form the future olfactory pits 

 may sometimes be seen at this period. 



The notochord is completely delaminated except in the region of 

 the primitive streak, by the time the neural tube has closed. 



By the time the neural tube is completed, the archenteron is called 

 a mesenteron, the anterior enlarged end forming the fore-gut, the walls 

 of which are but one cell in thickness (Fig. 329). This is also called the 

 pharynx in the embryo. The stomach and oesophagus are later to be 

 developed from this region. 



Fig. 329. 



Sagittal section of Anterior end of a frog tadpole 3.6mm. 

 .ong. (Redrawn from Corning.) 



