THE DETERMINATION OF SEX 



91 



transformation of the sexual cells. Fig. 36 shows a group of 

 cells, the nuclei of which have assumed the spindle form. We 

 see clearly the fibers of the spindle and the chromatine col- 

 lected in the middle of each spindle. The chromatine con- 

 sists of chromosomes which lie crowded together. The re- 

 maining pictures which we have to consider represent merely 

 the nuclei. Fig. 37 shows the successive alterations which the 



Fig. 38. — Anasa tristis. Spermatocyte nucleus in preparation for the first. 

 division, x, the accessory chromosome; p, the plasmasome, a transitory structure, 

 which does not belong to the chromosomes, but soon dissolves. 



nucleus of a sexual cell passes through when it begins to trans- 

 form itself into a sexual element. Soon an accessory chromo- 

 some becomes distinct, especially in the stages shown in 

 Fig. 38, during which the chromosomes become again dissolved 

 except the accessory, which behaves independently and inain- 

 tains its integrity. The accessory chromosome has no ab- 

 solute constant form, but varies greatly. Many of these 

 variations have been pictured by Miss Pinney. Fig. 39 

 leads us to the first development of the sexual ceU (first 

 spermatocyte). We recognize easily the spindle figure. 

 Out of the dissolving skein of chromatine complete chromo- 

 somes have arisen. The accessory chromosome lies always 

 at the side of the others. AU the chromosomes divide, and 

 we can observe readily in the figures how the two groups of 

 chromosomes diverge and move toward the poles of the 

 spindle. In each group there is one chromosome which has 

 been formed by the division of the accessory. The four draw- 

 ings in the lower part of Fig. 39 from right to left illustrate the 



