724 CALKINS. [Vol. XV. 



itself begins to divide. At first this is indicated by a slight 

 depression in the center of the nucleus (Fig. 20). The daughter- 

 nuclei then move further apart until the connecting-piece is 

 reduced to a mere thread, which soon breaks. The membrane 

 reforms, and all traces of the groove are obliterated as each 

 daughter-nucleus finally rounds out. The chromatin of the 

 daughter-chromosomes fuses to form again the great karyosomes 

 of the resting nucleus (Fig. 25), probably by a reversal of the 

 process of disintegration described above. The distal elements 

 of the daughter-chromosomes are the first to disappear, while 

 the thicker parts remain as the last evidence of division (Fig. 

 25). In some instances the karyosomes begin to form before 

 the daughter-nuclei are separated, as in Fig. 20. 



In the spore-forming divisions, on the other hand, which 

 follow each other in quick succession, the chromosome-structure 

 is not altered, and the daughter-chromosomes pass directly into 

 the nuclear plate of the next division figure. This process is 

 repeated nine or ten times until all of the spores are formed. 

 Fig. 21 represents the beginning of a second division before 

 the first is quite complete, and with a relatively large " Verbin- 

 dungsstiick " left between the nuclei. The daughter-chromo- 

 somes which are to form the nuclear plate of the second division 

 are single (Fig. 30), and yet just before the second division they 

 are double (Fig. 38). This division must take place while the 

 nuclear plate is forming. Ishikawa states that chromosomes 

 are quadruple in nuclei destined to form spores, but I have 

 never found the chromosomes in the nuclear plate otherwise 

 than single or double. Even were they quadruple in the parent 

 nucleus it would not suffice to explain the eight or nine subse- 

 quent divisions which the nuclei undergo before the spores are 

 formed. The elements composing the chromosomes must 

 divide longitudinally before each division of the nucleus. As 

 there is no time for growth of the chromosomes between 

 divisions, the quantity of chromatin in the nucleus is constantly 

 reduced by half, and the nuclei become smaller and smaller. 



