THE CHROMOSOMES 115 



come scattered in the region between the others that 

 have retreated toward the poles. When the division 

 is completed the belated chromosomes are found to 

 be excluded from the daughter nuclei. They appear 

 irregular in shape and show signs of degeneration. 

 At the next division of the egg they may still be found, 

 but they are lost later, and seem to take no part in the 

 development. The difference between this and the 

 other cross seems directly caused by the differences 

 observed in the behavior of the chromosomes. 



A count of the chromosomes in the hybrid embryos 

 shows about twenty-one chromosomes. The mater- 

 nal nucleus contained eighteen. It appears that only 

 three of the paternal chromosomes have taken a 

 regular part in the development fifteen of them must 

 have degenerated in the way described above. The 

 hybrid embryos that developed were often abnormal; 

 the few that developed as far as plutei were apparently 

 entirely maternal in character. Since the reciprocal 

 cross proves that the maternal characters are not 

 dominant, the most reasonable interpretation is that, 

 although the foreign sperm had started the develop- 

 ment, it had produced little or no effect on the char- 

 acter of the larvae, and this absence of effect would 

 seem most probably to be due to the elimination of 

 most of the paternal chromosomes. It might pos- 

 sibly be maintained that the same kind of effect pro- 

 duced by the egg of Strongylocentrotus on the chro- 

 mosomes of SphaBrechinus is likewise produced on the 

 protoplasm introduced by the sperm. But there is 

 here, in contrast to the case for the chromosomes. 



