REPRODUCTION AND DEVELOPMENT IN CHORDATES 



placed by others recently lormccl l)y mitosis from stocks of relati\ely un- 



specialized cells. Cells that do not become differentiated during the 



developmental period are known as totipotent or embryonic cells in the 

 adult animal. 



Metabolic Requirements of Embryos 



The orderly series of changes which occurs during development depends on 

 chemical reactions that take place in the cells, and these, in turn, on the 

 metabolic requirements of the cell. A constant supply of food and oxygen 

 must be available, and waste products of metaloolism must be eliminated. 

 The temperature, which is one factor conditioning the rate of metabolism, 

 cannot vary widely, and drying must be prevented. 



In the frog embryo, which has been used to illustrate the course of develop- 

 ment in vertebrates, these metabolic requirements are met simply. A large 

 amount of food is stored in the egg, the egg is laid in water from which oxygen 

 is obtained by diffusion, and the breeding season occurs during a period of 

 the year when temperature conditions are favorable for development. The 

 frog embryo within the fertilization membrane is protected from adhesions, 

 and there are no so-called embryonic membranes. As the localization of the 

 primordia of organ systems occurs, a U-shaped sucker appears on the ventral 

 surface of the head of the frog embryo, and a median posterior extension 

 foreshadows the formation of a tail. The embryo hatches by slipping out of 

 its disintegrating jelly envelopes and becomes attached to objects in the water 

 by means of the sucker. When the stomodeum becomes continuous with the 

 pharynx and the tail is developed further, the individual swims and feeds, 

 although it does not resemble an adult frog. Such a self-supporting but not 

 fully developed individual is called a larva; the larva of the frog is known 

 as a tadpole. During the tadpole stages the frog feeds upon plants and 

 carries on gas exchange by means of its gills, of which there is first an ex- 

 ternal and then an internal set. Toward the end of the tadpole period in 

 development the animal begins a metamorphosis, or change from larval to 

 adult structure. Hind legs and then front legs appear and grow, the tail 

 decreases in size until it disappears, and the mouth and jaws become like 

 those of the frog. The lungs become functional, and the gills disappear. In 

 correlation with the change from a diet of plants to one of insects, the in- 

 testine becomes much shorter during the period of metamorphosis (p. 454). 

 Many frogs complete their metamorphosis about 3 months after hatching, but 

 the bullfrog usually passes its first winter as a tadpole. Such a larval period 

 in the life cycle of a vertebrate is unusual, although larval stages char- 

 acterize the development of many invertebrates (pp. 330, 375, 455, and 

 496). 



In fishes the telolecithal eggs are laid in water, but there is one important 

 difference in development as compared with that of the frog. Cleavage is 



159 



