400 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



Development of the Leopard Frog (Rana pipiens) 



Breeding Habits. — The eggs of the leopard frog are laid in the 

 spring soon after the animals emerge from hibernation. Temper- 

 ature seems to be an influential factor, since an early spring or 

 one that is cold and late correspondingly advances or retards the 

 date of spawning. Similar differences are observed in the period 

 of egg laying in shallow ponds that are easily warmed by the sun, 

 as compared with ponds and streams that are fed by springs and 

 hence are lower in temperature. Previous to the egg laying, the 

 male clasps the female tightly with his fore limbs, partially encir- 

 cling the anterior end of her body and using the hind limbs only to 

 help maintain the position on the dorsal surface of his mate. 

 Such a process of sexual union is known as amplexus. The indi- 

 viduals remain in this position throughout a period of many hours, 

 during which the eggs are extruded from the female and fertilized 

 in the water by spermatozoa emitted by the male. 



Reproductive Organs and Germ Cells. — It will be recalled 

 (Fig. 30, p. 52) that the ovaries of the frog are paired organs 

 attached by a mesentery to the dorsal face of the coelomic cavity, 

 and that the oviducts are convoluted tubes opening anteriorly to 

 the coelome and posteriorly to the cloaca. Internally, the ovaries 

 are divided into compartments (Fig. 210) from the walls of which 

 protrude the ova, surrounded by a single layer of follicle cells and 

 the thin layer of cells by which the ovum is attached to the wall 

 of the compartment. In their early stages within the ovaries, the 

 ova are small cells with a centrally placed nucleus. As growth 

 proceeds, yolk granules are deposited in the cytoplasm, particularly 

 on the side that will be the lighter-colored or vegetative portion 

 of the mature egg, and the nucleus becomes shifted to an eccen- 

 tric position in the animal portion. While these changes are 

 taking place the surface of the animal hemisphere becomes dark- 

 ened by the deposition of pigment in the outermost region of its 

 cytoplasm. In this manner the egg acquires, while still within the 

 ovary, a " polarity " along an axis connecting the centers of its 

 two hemispheres, so that one speaks of the animal and vegetative 

 " poles " and their respective " hemispheres." This polarity 

 acquired by the ovum within the ovary is believed to be definitely 

 related to the body axes of the future animal (cf. Fig. 213). 



At the time of ovulation, or discharge of eggs from the ovary, 



