Studies on Chromosomes 1 55 



maturation-divisions and in those of the spermatogonia; and it 

 also appears in the number and size of the components of the 

 chromosome-nucleoli when these can be distinctly recognized. 

 Figs, I and 2 illustrate this correlation and epitomize the most 

 essential facts. These figures have been selected from a much 

 larger number to show the clearest and most typical conditions. 

 Some of them are enlarged from the photographs reproduced in 

 Plate I. Many others, with an account of secondary variations, 

 are given beyond. Each horizontal row of figures represents 

 three stages of the same type which, with two exceptions, are all 

 from the same individual. The left hand figure in each row shows 

 the typical arrangement of the chromosomes in the metaphase 

 of the first spermatocyte-division, the middle figure a spermato- 

 gonial group, and the right hand one a nucleus from the growth 

 period, to show the chromosome-nucleolus together with some of 

 the diffused ordinary chromosomes. 



Fig. I, a-c (terminaUs, No. 3), represent these three stages in 

 an individual of the 21-chromosome type (Montgomery's material) 

 showing II chromosomes in the first division, 21 in the sper- 

 matogonia, and a single chromosome-nucleolus in the growth 

 period. (Additional figures of this individual in Fig. 3.) Fig. i, 

 d-f (terminalis. No. 19), show the 22-chromosome type, with 

 a small idiochromosome present in addition to the large one. 

 The small idiochromosome (/) is distinguishable in Fig. i, e. 

 (Additional figures in Figs. 4-6.) 



Fig. I, g-i (terminalis. No. 20), show the 23-chromosome type, 

 with one large supernumerary. In the spermatogonial group {h) 

 this chromosome and the small idiochromosome are probably rep- 

 resented by the two designated as / and s. The nucleus from the 

 growth-period (/), shows the plasmosome {p) and a tripartite 

 chromosome-nucleolus formed by the idiochromosomes and the 

 supernumerary attached in a row [cf. Photo 27; additional figures 

 in Figs. 7-8). Fig. I, /-/ (terminalis. No. 43), show a 23-chro- 

 mosome group with one small supernumerary. This clearly 

 appears in the spermatogonial group {s); and the small idiochro- 

 mosome {i) is also distinguishable. In the nucleus from the 

 growth-period (/), the supernumerary and small idiochromosome 



