xiv] PARTIAL FERTILISATION 209 



hybrids, and it was further observed that the nuclei on the 

 purely maternal side were much smaller than those on the 

 side with the hybrid skeleton. BOVERI has shown that in 

 Echinoids the size of the nucleus is generally proportional 

 to the number of chromosomes, and it may therefore be 

 assumed that the half of the pluteus with maternal skeletal 

 characters and small nuclei is derived from a blastomere 



/'0\ 

 O 

 O 







O ' 

 O 



M 



o i 



O ; 



CP 



O ', 



\ o ; 



Q 



FIG. 27. A "partially thelykaryotic " pluteus from the cross 

 Sphaerechinus 5 X Strongylocentrotus $ . The left side of the 

 pluteus has the skeleton of the paternal (Strongylocentrotus} 

 type ; on the right the skeleton is intermediate but more 

 nearly resembling Sphaerechinus. ouj represents small nuclei 

 from the ciliated band of the left (paternal) side ; c w 2 corre- 

 sponding larger nuclei of the right side. (From HERBST, 1 907.) 



with a purely maternal nucleus and the reduced number of 

 chromosomes, and the hybrid half from the other blastomere 

 in which the nucleus conjugated with the sperm nucleus. 

 The only objection to this conclusion is that larvae with 

 asymmetrical skeletons are common in the hybrids of these 

 species, but if HERBST'S "partially thelykaryotic" larvae 

 were simply examples of this asymmetry, the difference in 



D. C. 14 



