DEVELOPMENT OF THE IMPREGNATED EGG. 621 



As development goes on (Fig. 180), the head becomes larger, and 

 shows traces of the organs of special sense. The tail also increases in 



FIG. 178. 



FIG. 179. 



FIG. 180. 



Diagram of FROG'S EGG, in an early stage of de- EGG OF FROG, in process of development, 

 velopment; longitudinal section. 1. Thickened 

 portion of external blastodermic layer. 2. An- 

 terior extremity of the embryo. 3. Posterior 

 extremity. 4. Internal blastodermic layer. 5. 

 Cavity of vitellus. 



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 medulla oblon- 

 gata. In the mean time, the internal blastodermic layer, subsequently 



converted into the epithelium of the 

 intestinal canal, has been shut in by 

 the abdominal walls, and forms a closed 

 sac, of slightly elongated figure, with- 

 out inlet or outlet. Afterward, the 

 mouth is formed by a perforation 

 through both external and internal 

 layers at the anterior extremity ; while 

 a similar perforation, at the posterior 

 extremity, results in the formation of the anus. 



By a continuation of the same process, together with the develop- 

 ment of the intermediate vascular layer, the different portions of the 

 body are gradually constructed, producing the skeleton, the integument, 



FIG. 181. 



EGG OF FROG, farther advanced. 



TADPOLE, fully developed. 



the organs of special sense, and the muscles and nerves. The tail acquires 

 sufficient size and strength to be capable of acting as an organ of loco- 

 motion (Fig. 181). The intestinal canal is at first a short, wide, nearly 



