IV. i INITIAL STRUCTURE OF THE GERM 



263 



6. It remains for us now to consider the conclusions which 

 Boveri has drawn from the behaviour of eggs in which owing 

 to pathological mitosis the distribution of the chromatic elements 

 was irregular. 



The investigations of O. and R. Hertwig had previously shown 

 that if the eggs of sea-urchins were weakened by treatment 

 with poisons (strychnine, quinine, and others), then the vitelline 

 membrane which normally prevents the entrance of more than 

 one spermatozoon was not formed, that consequently several 



FIG. 160. Diagram of one case of irregular chromosome distribution 

 in a doubly fertilized egg ; a n 6, , c,, andrf, ; 2 , Z>. 2 , c. 2 , and d. 2 , and 3 , 6 3 , 

 c s , and d 3 are the three complete, specific sets of unlike chromosomes. 

 (After Boveri, 1904.) 



spermatozoa entered, with the resulting formation of multipolar 

 mitoses and irregular cleavage. 



Boveri has succeeded in inducing dispermy in the eggs of 

 sea-urchins without the use of the poisonous reagent, by simply 

 adding an excess of sperm to the eggs. The number of ab- 

 normal larvae found in such a culture is greater than that 

 observed when the quantity of spermatozoa used is less, and in 

 proportion to the number of dispermic fertilizations. 



In dispermic eggs there is an extra pair of centrosomes, and 

 these two usually form with the other two a quadripolar figure 

 with four spindles occupying the four sides of the square (Fig. 160). 

 In the equators of these four spindles are placed the chromosomes, 



