STUDIES IN CYTOLOGY 397 
develop very irregularly, pronounced abnormalities appearing 
during the blastula stage. 
The correlation between development and the behavior of the 
chromosomes is very striking. In a few instances the mitosis of 
the first segmentation is regular, with very little elimination of 
chromosomes. In the large majority of instances the behavior 
of the chromosomes is irregular from the beginning, many of the 
chromosomes being massed together and eliminated from parti- 
cipation in further mitosis. 
It has been suggested that the reason that the chromosomes of 
one species or the other lag during division in some crosses lies 
in the differences in normal division rate. It is interesting, in 
this connection that in the Arbacia ¢ « Toxopneustes <2 cross the 
Toxopneustes chromosomes lag although the division rate in this 
species is more rapid than in Arbacia. It is clear that the cause 
for lagging must be some other than that of difference in division 
rate. I have previously pointed out that in any Hipponoé-Tox- 
opneustes cross cleavage is hastened by the Toxopneustes sperm, 
1.e., that of the quicker species. 
ARBACIA 2 X TOXOPNEUSTES <. CROSS 
The chromosomes in the zygotes of Arbacia punctulata are 
in general short and rod-like in form. Owing to their extremely 
small size I have made no prolonged effort to make out individu- 
ality of form. Fig. 6 represents one of three sections passing 
througha mitotic figure in anaphase. Thetotalnumber of chromo- 
somes is about forty. There is some variation in length, the 
longest chromosomes in Arbacia exceeding in length the shortest 
in Toxopneustes. 
Figs. 10-17 represent anaphases of the first division of the 
crosses. These represent the most nearly normal figures that 
were found. 
Fig. 10 A, B and C represent the most normal division figure 
found. Of the Toxopneustes chromosomes the long rods and the 
V’s may be recognized. Counting all of the chromosomes present 
the formula would be 3¢; those lagging at the center probably 
would have failed to be included in the daughter nuclei. Most of 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, No. 3 
