258 GENERAL ZOOLOGY 



through the lower (inner) end of the choroid fissure and then 

 along the ventral side of the optic stalk to the brain. The 

 optic stalk itself gradually extends around the optic nerve to 

 produce a sheath for the latter. 



Metamorphosis. — The tadpole of Rana pipiens undergoes 

 metamorphosis, i.e., transformation into a frog, toward the end 

 of the third month of development. As the time nears, the 

 tadpole frequently comes to the surface and gulps air, which is 

 drawn into the lungs. The principal changes occurring during 

 metamorphosis are: (1) the final stages in the development of 

 the lungs, accompanied by the atrophy of the gills and the 

 closure of the gill slits; (2) the freeing of the forelimbs by the 

 rupture of the operculum, which disappears, and a marked 

 lengthening of the hindlimbs; (3) the enlargement of the stomach 

 and the shortening of the intestine; (4) the shedding of the larval 

 epidermis together with the horny jaws and the labial papillae; 

 and (5) the resorption of the tail. 



Law of Biogenesis. — The embryos of higher vertebrates often 

 display certain features, mainly anatomical, which seem to 

 represent some sort of recapitulation of structures found in lower 

 forms. Thus the structure and functions of the gills of the 

 tadpole and the plan of branchial circulation have their proto- 

 types in the corresponding organs of the fish. From this it is 

 not to be inferred that these organs are structurally identical, 

 because they are not; but the similarity in the general structural 

 plan of these organs in the two cases is very marked and the 

 function so far as the gills are concerned is the same in both. 

 These resemblances may not seem unusual since both the fish 

 and the tadpole live in an aquatic habitat and for that reason 

 require somewhat similar respiratory organs. However, the 

 fact that the embryos of the higher air-breathing vertebrates, 

 such as reptiles, birds, and mammals, which have no aquatic 

 larval period in development, also pass through stages in which 

 the branchial region resembles that of the fish in its general 

 anatomy, calls for some other explanation than that of environ- 

 mental requirements. 



To take the chick embryo of 72 hours' incubation for an 

 example (Fig. 161), the heart lies in an anteroventral position in 

 the body cavity and pumps the blood forward through a ventral 

 aorta (truncus arteriosus), from which it passes right and left 



