366 N. M. Stevens 



late prophase, two small spherical chromosomes and four larger 

 elongated ones are distinctly paired while the members of the 

 unequal pair (/z^, /zj) are separated. For a long time it was 

 impossible to be sure that an unequal pair was present, as fore- 

 shortening in the case of one chromosome (Aj) was possible, but 

 recently a comparatively large number of good spermatogonial 

 plates has been secured in which the inequality in length of one 

 pair is clearly demonstrated. No case has been found in which 

 the members of this pair appeared to be equal. Figs. 58, 59 and 

 60 show exceptionally clear cases, and Fig. 61 shows a peculiar 

 folding of the chromosome h^, whose significance may be apparent 

 as we proceed to consider the unequal heterochromosome bivalent 

 of the first spermatocyte. 



In Drosophila the heterochromosomes cannot be demonstrated 

 in the growth stages of the first spermatocyte. In some sections 

 from Hermann material stained with thionin the plasmosome 

 (p) and some of the chromosomes appeared as in Fig. 62 in cysts 

 adjacent to the spermatogonial cysts. In later growth stages 

 nothing definite, except the immense plasmosome, can be made 

 out in regard to the contents of the nucleus. The earHest pro- 

 phase of division is the appearance of the chromatin massed 

 together, usually on one side of the nucleus, while the plasmo- 

 some may be in the middle or on one side of the nucleus (Fig. 63). 

 In aceto-carmine preparations the chromosomes first appear in 

 early prophase, scattered through the nucleus, faintly stained and 

 irregular in outline (Fig. 64). The plasmosome may be broken 

 up at this time or it may appear intact in the spindle. Figs. 65 and 

 66 are later prophases in which the chromosomes are completely 

 condensed. The unequal heterochromosomes are h^ and h^. 

 Fig. 67 shows the three equal bivalents, and the unequal hetero- 

 chromosome pair in its simplest form, in the metaphase of the 

 first spermatocyte mitosis. Fig. 68 shows slight modifications of 

 this form from other cells of the same cyst. The most common 

 form of this pair is seen in Figs. 69 and 70, where there are two 

 equal V-shaped elements and a third portion (.v) which in many 

 cases looks like a separate element, and for a time the group was 

 thought to be trivalent; i. e., made up of two equal V-shaped 



