28 M. LOUISE NICHOLS. 



dence for this is further strengthened by the manner in which 

 the chromosomes develop from the resting nucleus. The resting 

 nucleus is a network formed by the gradual lengthening and an- 

 astomosis of threads which retain their individuality until just 

 before the formation of the nuclear membrane. From it arise the 

 chromosomes of the first maturation division by a breaking apart 

 of the threads of this network and their gradual shortening and 

 thickening. 



The question presents itself whether these relations of form and 

 position of the chromosomes are peculiar to Oniscns or character- 

 istic of the land isopods as a group. Two other genera I have 

 examined, Porcellio and Armadillo, The spermatogenesis of these 

 is similar to that of Oniscns, but the cells are smaller, in Armadillo 

 so small as to make an examination of the chromosomes very 

 difficult and uncertain. 



The cells of Porcellio more nearly approach those of Oniscns 

 in size and I was able to compare the prophase of the first division 

 in the two animals. Although the chromosomes of Porcellio are 

 smaller than those of Oniscns, the peculiarities of shape and 

 arrangement are similar. In Porcellio also are present two rings 

 lying opposite each other with a crescent between. More than 

 one crescent in a nucleus, however, I have not seen. One at least 

 of the straight chromosomes appears considerably smaller than 

 the others and, as in Oniscns, frequently lies not far from the 

 crescent (Figs. 5 and 6). I was not able to determine the num- 

 ber of chromosomes in Porcellio, as my preparations contained no 

 spermatocytic mitoses. Besides the smaller size of the chro- 

 mosomes would make an accurate determination even more diffi- 

 cult than in Oniscns. As far as can be judged from the appearance 

 of the prophase, the number would seem not to be very different 

 in the two genera. 



The presence of chromosomes of the three forms described 

 within the same germ nucleus is not then peculiar to Oniscns, but 

 probably is characteristic of the group, the genera being visibly 

 distinguished from each other by differences in size of the chromo- 

 somes and possibly by slight differences in the shape of homol- 

 ogous chromosomes. 



Comparing the results obtained for various animals and plants, 



