292 



This is the accessory chromosome undergoing an equation division. In 

 late telophase (Figs. 40—42) the accessory is intimately connected with 

 the mass of ordinary chromosomes and becomes unrecognizable. It 

 reappears again in the resulting spermatids (Figs. 43 and 44) lying in 

 a pale-staining reticulum as a sphere, rod or bilobed body again close 

 to the nuclear wall. Thus it is found in only one half of the sper- 

 matids. Figure 47 illustrates a spermatid of larger size and with two 

 accessory bodies. This increased size and double endowment of ac- 

 cessory chromosomes is probably the result of an amitotic nuclear 

 division in its primary spermatogonial ancestor. Such spermatids give 

 rise to giant spermatozoa which are occasionally seen. Later stages 

 in the metamorphosis of the spermatid into a spermatozoon shows a 

 steadily decreasing size, an elongation of the cytoplasm to form a blunt 

 tail and the growth of a delicate middle -piece from the nuclear wall 

 into this tail (Fig. 45). As the middle piece grows thicker and longer, 

 the nucleus grows smaller; and as the cytoplasmic tail elongates there 

 grows out into it from the middle-piece a slender filament around which 

 the cytoplasm ultimately disposes itself as a spiral fin. Figure 46 shows 

 a pair of young spermatids one with and the other lacking the ac- 

 cessory chromosome. The nuclear reticulum is clumped and pale. In 

 Figure 48 are shown a similar pair of spermatids at a later stage of 

 development. The head already shows a chromatic nuclear cap, but 

 the accessory body still persists in the vacuolated cytoplasm of 

 this structure. Still later stages show the gradual disappearance 

 (apparently by fragmentation and karyolysis) of the accessory chromo- 

 some. 



The facts as they obtain in Aplopus Mayeri agree in essential 

 points (except that there is here an absence of the accompanying micro- 

 chromosomes seen in some of the Hemiptera) with Wilson's latest 

 report on Anasa tristis. It is very gratifying that the doubt that had 

 temporarily been created in the minds of some cytologists during the 

 past year in regard to the accepted number of chromosomes in Anasa 

 tristis, and the actuality of a persisting accessory chromosome during 

 mitosis, has been so effectually dispelled by Wilson's recent decided 

 statements to the effect that after an extensive comparative study of 

 unfixed, smear, and fixed and stained preparations he has found no 

 evidence that compels a retreat from his former position concerning 

 the facts as he had previously reported them for Anasa tristis. 



If then the accessory chromosome is really a sex - determinant, 

 the bare general outlines of sex-production can be cogently stated in 

 Wilson's formula modified for Aploplus: 



