GENERATION. 133 







within the case throughout the winter, escapes the following 

 spring, to undergo the usual metamorphic change common 

 to dipterous insects. 



VERTEBRATA. Within the germinative area, which is 

 placed toward the dorsal surface of the egg, the germinal 

 membrane (blastodermic membrane) is formed, soon to 

 divide into an external and an internal layer. The former, 

 in which are subsequently found the organs of animal life, 

 namely, the nervous system, the muscles, the skeleton, etc., 

 has received the name of the serous, animal or nervous layer. 

 The latter, which gives origin to the organs of vegetative life, 

 and especially the intestines, is called the mucous or vegeta- 

 tive layer. By subdivision of the inner layer, a third or in- 

 termediate layer is formed, in which is developed the circu- 

 latory system. The first important subsequent change is 

 observed in the external layer, where within a translucent 

 space, a longitudinal (primitive) groove is formed, upon each 

 side of which a fold (dorsal lamina, dorsal plate) arises. 

 Such folds being converged join at the median line to con- 

 vert the groove into a canal (neural canal) subsequently to 

 contain the cerebro-spinal axis. Immediately beneath the 

 spinal groove, indeed formed within its lower wall, a peculiar 

 cartilaginous body, the ' chorda dorsalis,' is seen. The verte- 

 bral column is commonly developed within the sheath of this 

 structure, which, as a rule, soon thereafter disappears. While 

 these changes are being effected the animal layer also extends 

 laterally, partially, or entirely to enclose the yolk cavity. 



Two types of development: (1) aquatic, and (2) terrestrial. 



(1) Pisces. In cartilaginous fishes an osseous vertebral 

 canal never appears. With such the chorda dorsalis is per- 

 sistent. When an osseous fish is just escaped from the egg, 

 the remains of the vitelline sac, now called the umbilical 

 vesicle, are not yet absorbed, and often of large size; the gill 

 covers are not completely formed; the fins are without rays, 

 the mouth, which is placed transversely, is without teeth. 

 External or false gills are occasionally appended to the sides 

 of the head, as in Carcharias (white shark). 



Batrachia. The general resemblance in development to 



