306 



GERM-CELL CYCLE IN ANIMALS 



definite position on the spindle at this time, but, as 

 indicated in Fig. 82, the distribution of the maternal 

 and paternal chromosomes to the daughter cells is 

 entirely a matter of chance. If the homologous 

 maternal and paternal chromosomes really are dis- 

 tributed by chance to the eggs and spermatozoa 

 following synapsis, then the number of combinations 

 possible are as follows (Button, 1903) : 



The only direct evidence that such distribution 

 of chromosomes takes place is that furnished re- 

 cently by Carothers (1913) from a study of the 

 spermatogenesis of three Orthopterous insects, 

 Brachystola magna, Arphia simplex, and Dissosteira 

 Carolina. Miss Carothers, while working in Pro- 

 fessor McClung's laboratory, discovered a tetrad in 

 the first spermatocytes of these insects which consists 

 of two unequal dyads (Fig. 83) . During the two mat- 

 uration divisions the four parts of this tetrad pass 

 to the four spermatozoa, and consequently two sorts 

 of spermatozoa are produced so far as this chromo- 

 some is concerned, one-half with one of the larger 

 elements of the tetrad and one-half with one of the 



