VI 



ECHINODERM HYBRIDS 159 



Strongylocentrotus chromosome is still present. In regard to these three 

 chromosomes, therefore, there is no doubt that it is the male chromosomes 

 that are eliminated. There are other reasons, which cannot be gone into 

 here, for supposing that this is so in regard to all the fifteen chromosomes 

 which are got rid of. Thus the overwhelming influence of the female 

 parent in the formation of the pluteus skeleton is easily intelligible on 

 the hypothesis that the idioplasm is contained in the nucleus, since the 

 larval nuclei contain eighteen chromosomes from the female parent but 

 only about three from the male. 



Furthermore, the pathological phenomena in the blastulae owe their 

 origin to the same process. The eliminated paternal chromosomes do 

 not he altogether passive in the cytoplasm, but undergo repeated 

 fission, forming great masses of chromatin. At the formation of the 

 blastula these chromatin masses with surrounding cytoplasm are extruded 

 into the blastocoele, where they form the masses mentioned above. 

 In the cross Strongylocentrotus 9 x Arbacia ^ , which also results in plutei 

 with almost purely maternal characters, the paternal chromatin also 

 appears to be eliminated, but by an entirely different method. The 

 development of these larvae runs a somewhat similar course to that of 

 the Strongylocentrotus $ x Sphaerechinus ^ hybrids, only the patho- 

 logical phenomena in the blastulae make their appearance slightly later. 



An examination of the cleavage mitoses shows that there is here 

 no elimination of the chromosomes, all the expected 38 being present 

 at all stages up to the blastula (in Strongylocentrotus, w = i8 ; in Arbacia 

 M = 20). In correspondence with this we find that the size of the nuclei 

 (which is proportional to the number of chromosomes) is about the 

 same in the hybrid larvae as in those of the pure parent species 

 {Strongylocentrotus) . The nuclei in the pluteus stage, however, have many 

 fewer chromosomes (about 18, judging from direct counts), and corre- 

 spondingly their nuclei are smaller than those of pure bred nuclei, in the 

 proportion of about 21 : 36. Hence it appears that somewhere between 

 the beginning of the blastula stage and the pluteus an elimination of 

 about 20 chromosomes has taken place. Sections of the blastulae at the 

 time of the critical period of their development show numbers of nuclei 

 obviously pathological, others apparently in the process of extruding 

 chromatin into the cytoplasm, while still others have lying beside them 

 in the cytoplasm an extra-nuclear mass of chromatin. The conclusion 

 to be drawn from these observations is that the Arbacia chromatin is 

 eliminated from the hybrid nuclei at this stage. As in the case of the 

 hybrids described above, the blastocoele is filled up with these masses 

 of degenerating chromatin and their surrounding cytoplasm. 



While this explanation of the female prepotency in this cross is not 

 quite conclusive (for exceptions occur in the form of plutei, also purely 



