246 



BIOLOGY FOR BEGINNERS 



It develops in the embryo fish as an outgrowth from the throat, 

 extending back and enlarging into the present form, and often 

 losing all connection with the outer air. It is in precisely similar 

 manner that the lungs of all higher forms push out from the throat, 

 while retaining their connection with the mouth and performing 

 an entirely different function. Yet they are regarded as of like 

 origin and structure, so the lungs are homologous to the air bladder 

 of fishes, but by no means analogous (or like in function). 



In this connection it is interesting to note that in certain Aus- 



F 6 H 



FIG. 88. Embryonic development of fish. 



A, unfertilized egg; gd, germinal disc; y, yolk; B, zygote formed by union 

 of ovum and spermatozoon; C, D, cleavage; E, young embryo showing neural 

 groove at left; F, showing yolk nearly overgrown by the vascular membrane 

 (blastoderm) growing out from embryo; G, embryo with "yolk sac"; H, young 

 fish, just hatched, with yolk sac not yet absorbed. From Pearse. 



tralian fishes the air bladder is actually used as a lung and the gills 

 are poorly developed for breathing. 



As the development of higher forms goes on, the simple air 

 bladder becomes two lobed, its walls develop ridges, and finally 

 many-celled chambers which enormously increase the ulterior 

 surface. To the walls of these delicate cells a network of capillaries 

 brings the blood, and devices are provided to pump air in and out. 

 Thus from the air bladder of the fish, the lung of a bird or man 

 may trace its origin. 



