William J. Moenkhaus 47 



have to rotate through an arc of 90° in order that both kinds of chromo- 

 somes may again be halved. This rotation takes place between the 

 vesicular stage of the nucleus and the metaphase of the following divi- 

 sion. Just when during this period it takes place or mostly takes place 

 I cannot say. At the metaphase the rotation is probably completed. 

 When the rotation is completed both chromosome groups again occupy a 

 horizontal plane. 



In all but one of the cells of the Menidia hybrid examined the small 

 chromosomes bore a definite relation to the cleavage plane, that of a 

 position in the spindle away from the plane of division. In the single 

 exception it was found that the small chromosomes were above the large 

 ones and, hence, occupied a vertical plane. 



The behavior of the chromosomes in the third and subsequent cleav- 

 ages is different from that of the first two. It will, therefore, be advis- 

 able to describe these stages in detail before entering into a comparison 

 with the conditions found in other forms. 



8. Third Cleavage. — The two groups of chromosomes of the second 

 cleavage spindles pass into a resting condition of the four-cell stage, in 

 which it is again impossible to distinguish the two kinds of chromatin. 

 There is no constriction or partition to divide the nucleus into two lobes 

 or parts, as is common in Cyclops and Crepidula. Since, so far, the 

 chromatin in these hybrids had behaved essentially like that in the 

 other forms described by other authors, I expected that in the metaphase 

 and anaphase of the third division the short and long chromosomes should 

 again appear in two groups on the spindle. If the conditions here 

 should run parallel with the conditions in Cyclops and Crepidula, this 

 is what should be expected. But in this I was disappointed. Whereas 

 in the second cleavage every cell which I examined shows the two kinds 

 of chromosomes bilaterally distributed, the third cleavage spindles, for 

 the most part, do not show such distribution. An occasional spindle 

 occurs in which the grouping has not been completely destroyed. Figure 

 19 (Plate III), it will be seen, shows the short chromosomes to the rigtit 

 and the long ones largely to the left. It is to be noted, however, that each 

 kind is not restricted to its group but a few of each kind have become 

 mingled with those of the other. The mingled condition is the prevail- 

 ing type where it is impossible to make out any grouping. 



The position of the parental chromosomes with reference to the cleav- 

 age plane which could be so readily followed out during the second cleav- 

 age is during the third cleavage, naturally, largely destroyed since the 

 bilateral distribution of the chromatin has been destroyed. In three 

 spindles in which the position could be made out with reasonable cer- 



