MALE 



FROM SIMPLE CELL TO COMPLEX ANIMAL 



FlG. II. 

 PRIMORDIAL GERM CELLS 



33 



FEMALE 



%? j' UNIVA 



/' l\ SPERMAT 



LENT CHROMOSOMES i2X> 



SPERMATOGONIA OOGONIA 



BIVALENT CHROMOSOMES (X) 

 PRIMARY PRIMARY OOCYTE / */> 



SPERMATOCYTE 



SECONDARY 

 X)SPERMATIDS OOCYTES 



j-. i*-S UNIVALENT CHROMOSOMES iX) 



I MULTIPLICATION PERIOD 

 (MANY GENERATIONS; 



GROWTH PERIOD 



III. MATURATION PERIOD 



V. FERTILIZATION PERIOD 



V. CLEAVAGE PERIOD 



VI. PERIOD OF BODY-FORMATION 

 BY CELL DIVISION. AND SEG- 

 REGATION OF GERM CELLS 



PRIMORDIAL GERM CELL 



FIG. ii. A diagram of the germ cell cycle, suggesting the changes by which the primordial 

 germ cells in the body give rise to the mature eggs and sperm; fertilization; cleavage; and the segre- 

 gation of new primordial germ cells as the new body is developed. (See 40-52, 60.) The full 

 number of chromosomes in the cells of the body is 2x. The half number found at certain stages is 

 indicated by x. The large gastrula at the bottom of the figure is to illustrate the early differentia- 

 tion of germ and body cells. 



Questions on the figure. Trace the changes of the chromosomes through the 

 whole series of stages. The solid black is intended to emphasize the potential 

 germ cells. Why are all the cells of the two and four-celled stages represented in 

 black? Why are the chromosomes in the primary oocyte and spermatocyte said 

 to be bivalent? What is meant by univalent? What does "continuity of germ- 

 plasm" mean? 

 3 



