58 



REPRODUCTIVE SYSTEM OF THE ADULT FROG 



Normal nuclear growth cycle of the ovum of Rana pipiens. (Stage 1 ) Smallest 

 follicle in which the chromosomes within the germinal vesicle can be seen. 

 (Stage 2) The paired chromosomes are barely visible, embedded in a nucleo- 

 plasmic gel. Egg diameters less than 200 microns. (Stage 3) Eggs measuring from 

 200 to 500 microns in diameter, more detail visible through the transparent theca 

 cells. Lateral loop production begins. Zone of large irregular nucleoli may be 

 seen just beneath the nuclear membrane. (Stage 4) First development of yellow- 

 brown color and yolk. Eggs range in size from 500 to 700 microns in diameter. 

 Chromosomes attain length of about 450 microns. For salamanders of com- 

 parable stage chromosomes measure 700 microns in length. (Stage 5) Chromo- 

 some frame begins contraction while the nucleus continues to grow in eggs 

 ranging in diameter from 750 to 850 microns. This is approximately half the 

 ultimate size. Chromosomes shorten and have fewer and smaller loops. The 

 major nucleolar production continues and sacs appear on the surface of the 

 nuclear membrane. (Stage 6) Egg diameter about 1.8 millimeters and germinal 

 vesicle is of maximum size. Chromosome frame now about 1/1000 of the nuclear 

 volume, coated by a denser substance which can be coagulated by the calcium 

 ion. Chromosomes have shortened to 40 microns or less and have lost all large 

 and small hyaline bodies called loop fragments. 



Heavy arrows indicate the mixing of nuclear material in the cytoplasm after 

 the breakdown of the germinal vesicle. The dotted arrow indicates migration of 

 the central chromosomal mass toward the animal pole to become the maturation 

 spindle for the first polar body. (From W. R. Duryee, 1950, Antu N, Y, Acad. 

 Sci., 50, Art. 8.) 



