Studies on Chromosomes 85 



elongated idiochromosome is manifestly a single body. As it 

 shortens and condenses to form the chromosome nucleolus, it 

 shows a considerable variety of forms; and the rate of condensation 

 also varies, cells that are already entering the synizesis stage being 

 sometimes seen in which the idiochromosome is still distinctly a rod 

 (Photos 35 and 36). In most cases it is at this time a single 

 ovoidal or spheroidal body; but not infrequently it appears more or 

 less distinctly double (Photos 37 to 39). This condition is however 

 not produced by a previous synapsis of two chromosomes, as Gross 

 believed, but arises, I think, from a tendency of the chromatin to 

 accumulate towards the ends of the rod; and when this is very 

 marked it may assume an appearance of duality, even in the ear- 

 lier stages (Photo 37, below), though this is relatively rare. In the 

 later stages a double appearance is not infrequent, dumb-bell 

 forms being thus produced, which sometimes give in the synizesis 

 stage apparently double bodies. The earlier stages conclusively 

 show that this is a secondary appearance. In the later (postsyn- 

 aptic) stages (Photos 34 and 35), and throughout the growth 

 period, it always appears as a single spheroidal body. In view of 

 these facts I think the conclusion inevitable that the chromosome 

 nucleolus is a univalent chromosome that arises by the condensa- 

 tion of the unpaired large chromosome of the spermatogonia. 



I have little to add to Henking's and Gross's acounts of the 

 maturation divisions. As will be seen from Photos 45, 46, 50, 51, 

 the size relations are correlated with those of the spermatogonia! 

 chromosomes. In polar views of the first division appear with 

 great constancy four smallest chromosomes, one shghtly larger 

 one, and seven still larger ones, or twelve in all. The idiochromo- 

 some is one of the largest, but cannot be distinguished from the 

 others (as is also the case in Protenor). This is obviously due to 

 the fact that the idiochromosome is still a univalent or single 

 chromosome, while each of the others represents two of the sper- 

 matogonia! chromosomes united. Since all have nearly the same 

 dumb-bell shape as seen in side view, the idiochromosome appears 

 from the pole approximately but half as large, relative to the others, 

 as in the spermatogonia. The same size-relations appear in the 

 second division, but all the chromosomes are much smaller, as 

 the photographs clearly show. 



