148 THE CHROMOSOMES 
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 larve, 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 Spheerechinus is likewise produced on the 
protoplasm introduced by the sperm. But there is 
here, in contrast to the case for the chromosomes, 
