" HETEROTYPICAL MITOSIS IN NEREIS LIMBATA. 79 



mosomes ; and the first maturation division is certainly not a re- 

 duction division. 



The possibility, however, of the second maturation division of 

 Nereis being a reductional mitosis, is not absolutely excluded, 

 although the only reason for accepting this view must, as far as 

 I can see, be sought in a preconceived assumption in favor of 

 such a mode of maturation. 



The chromosomes of the prophase show in the second matura- 

 tion division, as in the first, and in the early cleavage divisions, 

 two longitudinal splits, one of which is identical with the split of 

 the daughter chromosomes of the first division, the other being 

 a new formation generally not appearing till in the prophase of 

 the second but sometimes indicated as early as in the anaphase 

 of the first division. In most cases it seems impossible to decide 

 with absolute certainty which of these splits represents the 

 division plane of the chromosomes. 



Supposed, however, that a separation of the conjugated chro- 

 mosomes should take place in this division, then the thorough 

 resemblance in the genesis and the whole appearance of the chro- 

 mosomes during the maturation and cleavage divisions must be 

 considered as a mere chance. The longitudinal split of the daugh- 

 ter chromosomes, would have a different meaning in each of the 

 maturation divisions and in. the cleavage; in the first maturation 

 division it would mean the split between the conjugated chromo- 

 somes, in the second it must be explained in some other way — 

 most likely, perhaps, as a precocious splitting for the next division ; 

 in the cleavage divisions, finally, the longitudinal split is cer- 

 tainly not to be seen in connection with the following division as 

 it is shown to be identical with the space between the two 

 branches of the V-shaped chromosomes of the prophase. In 

 the same way all the other stages in the development of the 

 chromosomes would, in spite of their detailed resemblance, have 

 to be explained in different ways. 



Considering, on the other hand, the fact that in this species, so 

 favorable for an examination of the chromosomes, no evidence at 

 all is found, which might establish a proof of the existence of a 

 reduction division — it seems to me more natural to look upon 

 the conformity in the behavior of the chromosomes as an expres- 



