726 



DEVELOPMENT OF THE IMPREGNATED EGG. 



are afterward developed the bodies of the vertebrae (Fig. 243,4), and. 

 the oblique processes of the vertebrae run upward from this point into 

 the dorsal plates, while the transverse processes and ribs run outward 

 and downward in the abdominal plates, to encircle more or less com- 

 pletely the corresponding portion of the body. 



In a longitudinal section of the egg, made while this process is going 

 on, the thickened portion of the external blastodermic layer (Fig-. 244, i) 

 may be seen in profile. The anterior portion (2), which will form the 

 head, is thicker than the posterior ( 3 ), which will form the tail. As the 

 whole mass grows rapidly, both in the anterior and the posterior direc- 

 tion, the head becomes thick and voluminous, while the tail begins to 

 project backward, and the egg assumes an elongated form. (Fig. 245.) 



Fig. 244. 



Fig. 245. 



Diagram of FROG'S EGO, in an early EGG OF FROG, in process of develop- 



stage of development; longitudinal sec- ment. 



tion. 1. Thickened portion of external 

 blastodermic layer. 2. Anterior extremity 

 of the embryo. 3. Posterior extremity. 4. 

 Internal blastodermic layer. 6. Cavity of 

 vitellus. 



The abdominal plates also meet upon its under surface, and complete 

 the closure of the abdominal cavity. The internal blastodermic layer 

 is seen, in the longitudinal section of the egg, embraced by the abdo- 

 minal plates, and inclosing, as before, the remains of the vitellus. 



As development goes on (Fig. 246), the head becomes larger, and 

 shows traces of the formation of organs of special sense. The tail also 



Fig. 246. 



EGG OF FROG, farther advanced. 



increases in size, and projects farther from the posterior extremity of 

 the embryo. The spinal cord runs in a longitudinal direction from front 

 to rear, and its anterior extremity enlarges, to form the brain and me- 

 dulla oblongata. In the mean time, the internal blastodermic layer, 

 which is subsequently converted into the intestinal canal, has been shut 



