158 



CYTOLOGY 



CHAP. 



X 



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majority die. Those that reach the pluteus stage are found to exhibit 

 exclusively the characters of the female parent {Strongylocentrotus). 



Cytological examination of these hybrids {Strongylocentrotus 5 x 



Sphaerechinus $) pro- 

 vides a satisfactory ex- 

 planation both of the 

 pathological course of 

 development and of the 

 purely maternal char- 

 acteristics of the plutei. 



In the anaphase of the 

 first cleavage division a 

 number of chromosomes 

 fail to travel up to the 

 poles with the other 

 chromosomes, but remain 

 lying in between the two 

 daughter groups (Fig. 72). 

 They fail to enter into 

 the daughter nucleus, re- 

 maining as extra-nuclear 

 masses of chromatin in 

 the blastomeres. A 

 further number of chro- 

 mosomes get left out in 

 the second cleavage 

 mitosis. The nuclei from 

 now onwards have only 

 about twenty or twenty- 

 one chromosomes (vary- 

 ing in different larvae 

 from nineteen to twenty- 

 four) instead of the 

 expected thirty-six {n in 

 both parent species being 

 eighteen). Thus about 

 fifteen chromosomes have 

 been eliminated. These 

 rejected chromosomes are, 

 in all probability, those brought in by the male gamete. For instance, 

 two pairs of the Sphaerechinus and one pair of the Strongylocentrotus 

 chromosomes are distinguishable by their shapes and sizes. After the 

 elimination we never find these two Sphaerechinus chromosomes, but the 



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36 



B 



Fig. 72. 



The chromosomes of the hybrid Strongylocenlrotus 9 ^ Sphaer- 

 echinus (5. (Baltzer, A.Z., 1910.) A, B, anaphase and telophase of 

 the first cleavage division. A number of chromosomes fail to enter 

 either daughter nucleus. C, 4-cell stage. The eliminated chromosomes 

 form irregular masses of chromatin between the daughter nuclei. 



