14 DEVELOPMENT OF THE FROG'S EGG [Ch. T 



polar body forms immedi?ite\j tvithout a reatincj-period. Further, 

 it is found, after the extrusion of the second polar body, that the 

 number of chromosomes in the Q^g is reduced to half the num- 

 ber characteristic for the somatic cells. Tetrads have also been 

 described as occurring just before the extrusion of the polar 

 bodies. In many cases the first polar body divides into two, 

 so that three polar bodies are present. These three polar bodies 

 and the egg seem to correspond to the four spermatozoa from 

 each spermatocyte. All four spermatozoa are functional, but 

 onlj^the egg (and not its three polar bodies) is capable of devel- 

 opment. Weismann has utilized the discovery of the reduction of 

 the number of chromosomes to build up an elaborate and highly 

 speculative theoiy of heredity. The reduction division is, ac- 

 cording to Weismann, not simply a quantitative division of the 

 chromatic thread, but is at one stage at least a qualitative 

 division. The reduction of the chromosomes to half the number 

 pi-esent in the other cells of the body seems, according to Weis- 

 mann and others, to be a preparation for fertilization. Since 

 the spermatozoon brings into the Qgg only half the number of 

 chromosomes found in the somatic cells of the animal, and 

 since the egg-nucleus supplies the other half, the number of 

 chromosomes will thus remain constant for the species from 

 generation to generation. 



