STUDIES IN CYTOLOGY 399 
Fig. 20 A and B, chromosome formula 7%. Almost a typical 
' Toxopneustes thelykaryotie figure. Possibly one or two Arbacia 
chromosomes present. 
Fig. 21 A and B, tripolar figure. Total number of chromo- 
somes present about 85. Four Toxopneustes V’s (two pairs) 
present. If this egg was fertilized by two Arbacia spermatozoa 
about half of the Arbacia chromosomes have been eliminated. 
I have already pointed out the fact that few of the Arbacia- 
Toxopneustes hybrids pass through the gastrula stage. From 
my experiments I have obtained very few plutei indeed, and 
these few plutei showed evidence of their hybrid origin. If we 
examine some of the more irregular division figures, not only of 
the first cleavage but of the later cleavages, we shall find many 
very pronounced abnormalities; we shall find that not only may 
the chromosomes of the sperm be involved but that those of the 
egg as well fail to take their places in the spindle, the result 
being that the full haploid number of neither parent is present. 
This fact may well be intimately associated with the abnormal 
development during the blastula stage. 
The only cytological investigation having immediate connec- 
tion with these crosses is that of Baltzer (10), on reciprocal 
Arbacia pustulosa crosses. -Baltzer has shown that the form of 
the chromosomes in this species is like those of Arbacia punctu- 
lata which I described in 1907. Baltzer made six Arbacia crosses. 
Strongylocentrotus ¢ x Arbacia 
Sphaerechinus ¢ xX Arbacia 7 
Kehinus ¢ X Arbacia 2 
Arbacia @ X Strongylocentrotus 
Arbacia ¢ X Sphaerechinus 
Arbacia ¢ X Echinus 2 
In all of these crosses the results are in general like those which 
I have just described for Arbacia punctulata. In all of the 
crosses, in most individuals, there is an ‘Erkrankung’ before or 
during the blastula stage, this indisposition being so marked as to 
resemble a sudden poisoning of the embryos. 
In Baltzer’s Arbacia ¢ by Strongylocentrotus ~ or Echinus ¢ 
crosses from 8 to 10 chromosomes were eliminated. In the Arba- 
