360 N. M. Stevens 



The testes (or ovaries) of adult flies were dissected out in physio- 

 logical salt solution and immediately transferred to a drop of aceto- 

 carmine on a slide. The cover-glass v^ras pressed down with a 

 needle to break the capsule of the testis and spread the cells. All 

 excess of stain was removed with filter paper, and after ten or 

 fifteen minutes, the preparation was sealed with vaseline. Such 

 preparations may be studied to the best advantage after twenty- 

 four hours, as the chromatin gradually acquires a deeper tint. 

 They remain in good condition for several days, but are, of course, 

 not permanent. The method has several advantages besides that 

 of enabling one to examine a large amount of material in a limited 

 time. The aceto-carmine fixes and stains instantly without the 

 shrinkage incident to the usual treatment with fixing fluids, alco- 

 hols, xylol and paraffin, necessary in order to obtain sections. 

 Then, one is able to study the whole cell with all of the chromo- 

 somes present and uncut, which is an obvious advantage for work 

 of this kind. The chromatin stains much more deeply than any 

 other cell element, but the achromatic structures are not always 

 well brought out, and they have been omitted from most of the 

 figures, as this investigation is concerned primarily with the hetero- 

 chromosomes and the method of synapsis. In favorable prepara- 

 tions of this kind, with good fight, it is possible to get as accurate 

 camera drawings as from sections stained with iron haematoxyhn. 



RESULTS OF INVESTIGATION 

 I Muse a domestica 



In many respects the spermatogenesis of this fly resembles that 

 of Tenebrio molitor (Stevens '05), Odontata dorsalis (Stevens 

 '06) and the other Coleoptera which have an unequal pair of 

 heterochromosomes. There are, however, no synapsis, synize- 

 sis or spireme stages in the spermatocytes, nor are tetrads ever 

 formed. 



In the prophase of spermatogonial mitoses one finds five pairs 

 of long slender chromosomes, the members of each pair either 

 lying parallel to each other or twisted together (Fig. i). The 

 members of the additional unequal pair are usually separate 



