420 VERTEBRATE LIFE AND ORGANIZATION 



193. Life Cycle 



A look at certain aspects of the frog's development is no less im- 

 portant than studying the adidt, for the continuation of the species re- 

 quires its reproduction, and the development of a reasonable propor- 

 tion of the fertilized eggs into adults of the next generation. Great 

 numbers of eggs must be laid by frogs and other animals that do not 

 care for them because the mortality of such eggs and young is very high. 

 The leopard frog lays from 2000 to 3000 eggs, and the bullfrog can lay 

 up to 20,000 per year. 



Eggs are laid in the spring, often in rather cold water. However, 

 development can proceed, for the pigmentation of the upper hemisphere 

 of each egg absorbs some heat, metabolic activity produces more, and 

 the jelly coats provide some insulation. The fertilized egg, or zygote, 

 cleaves systematically into progressively smaller cells during the early 

 stages of development, finally attaining the blastula stage, at which 

 time the embryo is a hollow sphere of cells (Fig. 21.20). Since its lower 

 cells contain more yolk and are larger, the cavity of the blastula, the 

 blastocoele, is excentric in position. 



This stage is followed by gastrulation, a dynamic process during 

 which the cells of the blastula that are destined to form the major 

 organs of the body are moved to appropriate regions of the embryo. 

 This involves the inward movement of many cells (p. 128), the elimina- 

 tion of the former blastocoele, and the formation of the primitive gut 

 cavity, the archenteron. The last temporarily opens to the surface 

 through the blastopore, an opening which is occluded to some extent 

 in frogs by a plug of yolk-laden cells, the yolk plug. 



Shortly after this, the embryo begins to elongate. A pair of lon- 

 gitudinal neural folds, destined to meet dorsally and close to form the 

 tubular nervous system, appears along its back, and the embryo begins 

 to acquire a distinct head, trunk and tail. A pair of oral suckers, for 

 later attachment, and primordia for the eyes and gills are evident upon 

 the head. Embryonic muscle segments (myotomes) form along the trunk 

 and tail, and the heart begins to beat. 



About this time the embryo wriggles out of its jelly capsule and 

 hatches into a free-swimming larva, or tadpole. Nasal cavities, finger- 

 like external gills, and mouth and cloacal openings soon appear, and 

 the larva can take care of itself. Most frog tadpoles feed upon minute 

 plant material, scraping it up with horny teeth. The younger tadpoles 

 attach onto the plants on which they are feeding by means of their 

 oral suckers. Plant inaterial is more difficult to digest than animal matter, 

 and plant-eating vertebrates generally have longer intestines, which 

 provides more digestive and absorptive surface, than their carnivorous 

 relatives. The intestine of a tadpole is many times the length of the 

 body and is coiled like a watch spring. 



Later in larval life, the external gills become covered by the growth 

 of a fold known as the operculum, and gill slits develop that lead from 

 the pharynx to the opercular chamber. About this time the external 

 gills are lost and the larvae respire by internal gills that develop within 



