8 



DEVELOPMENT OF THE FROG'S EGG 



[Ch. I 



vom Rath has found that at the end of the third generation 

 large cells appear with huge nuclei (Fig. 5, B), in which there 

 are twelve groups of chromosomes. Each group or tetrad is 

 composed of four granules. There are, therefore, present forty- 

 eight spherical chromosomes united in groups of four. These 

 tetrads arose from a heterotypic spindle, and in the following- 

 way. As the twelve loops (which are now double, making 

 twenty-four loops) passed toward one pole they became much 

 thicker (Fig. 5, A). The middle point of union of each of the 

 twenty-four loops broke (Fig. 5, B), and the portions rounded 

 up, so that there were present forty-eight chromosomes arranged 

 in twelve groups of four chromosomes each (Fig. 5, B). Imme- 

 diately after the formation of the tetrads the groups of chromo- 

 somes arranged themselves along the rays of the achromatic 



A B C 



Fig. 5. — Formation of tetrads in testis of Salamandra. (After vom Rath.) 



spindle (Fig. 5, C). The tetrads next passed toward the 

 equator of the spindle, and there they divided, so that two of 

 each of the four chromosomes passed toward one pole of the cell 

 (as in Gryllotalpa). In this way two new cells are formed 

 with twenty-four chromosomes each. A second division suc- 

 ceeds without an intervening resting-stage, and the number of 

 chromosomes is reduced, so that each cell has twelve chromo- 

 somes. The cells resulting from the last division, having each 

 twelve chromosomes, differentiate each into a spermatozoon. 



The second division, according to some workers (Boveri, 

 Hertwig, and Brauer), is the result of a second longitudinal 

 division. But vom Rath holds that this second division in the 

 Amphibia and in Gryllotalpa is the result of a cross-division of 



