60 HEREDITY AND SEX 



number is found to be about twenty-one. The maternal 

 egg nucleus contains eighteen chromosomes. It appears 

 that only three of the paternal chromosomes have 

 succeeded in getting into the regular cycle fifteen of 

 them have degenerated. 



Baltzer thinks that the egg acts injuriously in this 

 case on the chromosomes of foreign origin, especially 

 on the fifteen that degenerate, so that they are elim- 

 inated from the normal process. 



The embryos that develop from these eggs are often 

 abnormal. A few develop as far as the pluteus stage, 

 when a skeleton appears that is very characteristic for 

 each species of sea-urchin. The plutei of these hybrids 

 ~are entirely maternal. This means that they are 

 exactly like the plutei of the species to which the 

 mother belongs. 



The conclusion is obvious. The sperm of sphserechi- 

 nus has started the process of development, but has 

 produced no other effect, or has at most only slightly 

 affected the character of the offspring. It is reason- 

 able to suppose that this is because of the elimination 

 of the paternal chromosomes, although the evidence 

 is not absolutely convincing. 



Let us now examine the reciprocal cross. When the 

 eggs of sphserechinus are fertilized by the sperm of 

 strongylocentrotus, the division of the egg and of the 

 chromosomes is entirely normal. All the chromosomes 

 divide and pass to the poles of the spindle. The total 

 number (36) must, therefore, exist in each cell, although 

 in this case they were not actually counted. 



The pluteus that develops has peculiarities of both 

 maternal and paternal types. It is hybrid in structure. 



