400 : DAVID H. TENNENT 
cia ° by Sphaerechinus ~ cross about 18 chromosomes were 
eliminated (Baltzer ’10, text figs. 18 and 19). In Baltzer’s © 
Echinus 2, Strongylocentrotus ¢ and Sphaerechinus ¢ x Arba- 
cia & crosses two types of behavior were apparent. In the first 
two cases elimination of chromosomes occurred during the blastula 
stage; in the last case the chromosomes were retained. In all of 
these cases the skeleton of the plutei was nearer the maternal 
type than the paternal, but the hybrid nature was evident. 
My Arbacia ¢ xX Toxopneustes ~ cross is almost identical in 
its behavior to Baltzer’s Arbacia ¢ x Echinus ¢ or Strongylo- 
centrotus # cross. My Toxopneustes ¢ x Arbacia ¢ cross is 
unlike any of Baltzer’s Arbacia ~ crosses, in that elimination of 
chromosomes took place during: the cleavage. I shall return to 
these points later. 
SUMMARY 
The results of this investigation may be summarized as follows: 
1. The study of artificially parthenogenetic eggs has shown 
that the eggs of Toxopneustes are all alike in that each contains 
two V-shaped chromosomes. 
2. The study of fertilized enucleated fragments of Toxopneustes 
eggs has shown that Toxopneustes spermatozoa are of two classes, 
one class containing one V-shaped chromosome, the other contain- 
ing two-Vshaped chromosomes. 
3. The study of the Arbacia ¢ xX Toxopneustes < cross has 
shown that elimination of both Arbacia and Toxopneustes chro- 
mosomes may take place during cleavage. 
4, The study of the Toxopneustes ¢ x Arbacia ~ cross has 
shown that nearly all of the Arbacia chromosomes may be elim- 
inated in the early cleavage. 
DISCUSSION 
The first point requiring consideration is in connection with the 
results of the study of parthenogenetic eggs and of fertilized enu- 
cleated egg fragments of Toxopneustes. I have shown here that 
Toxopneustes is in agreement with Hipponoé (711, 712), in that the 
