64 



REPRODUCTIVE SYSTEM OF THE ADULT FROG 



and phospholipin content of the ovaries of Rana pipiens occurring 

 during the production and growth of ova (Boyd, 1938). The primary 

 oocyte may show a sHght flattening of the surface directly above 

 the region of the nucleus. 



These growth changes and the unequal distribution of pigment, 

 yolk, and cytoplasm are the first indications of polarity or a gradient 

 system within the egg. When the polarity is well established, the 

 cytoplasm, the superficial melanin or black pigment, and the nucleus 

 are all at the animal hemisphere (pole). The light colored yolk is 

 more concentrated toward the vegetal pole. The egg is then re- 

 garded as a telolecithal egg. During this phase of egg maturation 

 there is a drain on the metabolism of the frog which requires an ex- 

 cess of food intake because the materials for egg growth must be 

 synthesized from nutritional elements received from the vascular 

 system of the female. For Rana pipiens this period of most active 

 feeding comes during the summer when the natural foods, insects, 

 worms, etc., are the most abundant. 



During the growth of the oocyte in general there are important 

 changes occurring within the nucleus (germinal vesicle) of the egg. 

 Thirteen pairs of chromosomes may be 

 seen in synizesis (contraction), converg- 

 ing toward the centrosome at the "yolk 

 nucleus" stage. A little later the nuclear 

 membrane develops sac-like bulges, the 

 nucleoli are scattered, and there is a 

 colloidal chromosome core which almost 

 fills the entire nucleus. The chromo- 

 somes themselves are small and almost 

 invisible. When the primary oocyte is 

 about half its ultimate size, there appear 

 definite sacs on the nuclear surface. The 

 fragmented nucleoli are located at the 

 periphery of the lobulated nuclear mem- 

 brane, and the chromosome frames have 

 become relatively large. The chromo- 

 somes, by this time, have reached their 



maximum length and possess large lateral loops. Finally, in the fully 

 grown nucleus of the primary oocyte the nuclear sacs are very 

 prominent, and the nucleoli appear in clusters in the center of the 



Diploid metaphase chromo- 

 somes from the tail fin of a 15- 

 day-old Rana pipiens tadpole. 

 (Courtesy, K. R. Porter, 1939, 

 Biol. Bull., 77:233.) 



