5 in 



THE AMERICAN NATURALIST [Vol. XLIV 



of Sptrorjijra lie in the substance of the large nucleole. 4 

 He was able by the proper extraction of stain to differ- 

 entiate fourteen bodies in the gamete nucleus which cor- 

 responded in number to the fourteen chromosomes that 

 may be readily counted for this species in later mitoses. 

 The union of the gamete nuclei in the zygospore involves 

 the fusion of the two nucleoles to form a dense homo- 

 geneous structure iu preparation for a characteristic as- 

 sociation of the two sets of chromosomes, one derived 

 from each parent. 



The approach of the first mitosis is indicated by 

 changes within the fusion nucleole where a granular 

 structure begins to appear, accompanied by irregulari- 

 ties in its form. As these changes proceed an assemblage 

 of deeply staining bodies becomes evident which in later 

 stages are readily identified as chromosomes. There are 

 twenty-eight of these grouped in fourteen pairs, an ar- 

 rangement which is characteristic of a phase of chromo- 

 some reduction that follows synapsis and which signifies 

 that the chromosomes are to be distributed in two sets 

 of equal numbers by the following mitosis. This dis- 

 tribution occurs in Spirogyra and the second mitosis in 

 the zygospore is concerned with the reduced, or haploid, 

 number of chromosomes. Thus the double, or diploid, 

 number is present only in the fusion nucleus; the numer- 

 ical reduction takes place with the first mitosis. There 

 are no nuclear divisions in which the diploid number of 

 chromosomes is passed on, and assuming that the sporo- 

 phyte generation is characterized by nuclear divisions 



