592 



DEVELOPMENT OF THE IMPREGNATED EGG. 



meet upon its under surface, and the point at which they finally 

 unite forms the abdominal cicatrix or umbilicus. The internal blas- 

 todermic layer is seen, of course, in the longitudinal section of the 



Fig. 198. 



Tig. 199. 



Diagram of FROG'S EGG, in an earl.v Eoo OF FBOQ, in process of develop- 



stage of development ; longitudinal sec- meat, 



tion. 1. Thickened portion of external 

 blastodermic layer, forminv body of fetus. 

 2. Anterior extremity of foetus 3 Poste- 

 rior extremity. 4. Internal layer of blas- 

 todermic membrane. 5. Cavity of vitellus. 



egg, as well as in the transverse, embraced by the abdominal plates, 

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



As the development of the above parts goes on (Fig. 200), the 

 head becomes sill larger, and soon shows traces of the formation 



Fig. 200. 



EGO OF FROG, farther advanced. 



of organs of special sense. The tail also increases in size, and pro- 

 jects 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, so as to form the brain and medulla 

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

 is subsequently to be converted into the intestinal canal, has been 

 shut in by the abdominal walls, and still forms a perfectly closed 

 sac, of a slightly elongated figure, without either inlet or outlet. 

 Afterward, the mouth is formed by a process of atrophy and perfo- 

 ration, which takes place through both external and internal layers, 



