372 A^. M. Stevens 



in Lucilia caesar, Calliphora vomitoria and Sarcophaga carnaria 

 is approximately equal, and his experiments show that neither 

 amount nor kind of food given to the larvae has any marked effect 

 on the proportion of the sexes in the first or second generation, but 

 here as elsewhere in such experiments the number of eggs that 

 did not hatch is not noted, and this may be the critical point. 

 It is evident that more experiments are needed in which the fate 

 of all of the eggs of isolated pairs of flies is determined. 



2 Synapsis 



In the spermatogenesis of most insects synapsis involves an 

 end-to-end union of homologous chromosomes, and tetrads of 

 various forms are commonly found in the prophase of the first 

 spermatocytes. In these flies no tetrads have been observed and 

 as a rule nothing comparable to the synizesis, bouquet or spireme 

 stages of other forms is apparent. In these respects the germ 

 cells of the Diptera resemble the oogonia of sagitta (Stevens '03 

 and '05) and the male and female germ cells of the aphids (Stevgns 

 '05 and '06). In the oogonia of Sagitta the chromosomes pair 

 side-to-side in an early stage, while in the spermatogonia of the 

 aphids the pairing occurs as a prophase of the first spermatocyte 

 mitosis. The indications are that in the flies the chromosomes 

 are already paired side-to-side at the beginning of the growth 

 stage (Figs. 87 and 88), but the pairs do not appear to unite end- 

 to-end to form a spireme. In some cases the members of the pairs 

 are perfectly fused in the prophase of the first spermatocyte (Figs.. 

 3 and 27); in others the bivalents are clearly such in both prophase 

 and metaphase (Figs. 44 to 46). The first spermatocyte division 

 is without doubt reductional for both ordinary chromosomes and 

 heterochromosomes. 



Perhaps the most interesting point in the whole study is the 

 pairing of the chromosomes in cells somewhat removed from the 

 sphere of the reduction process. This was first noticed in the 

 oogonia of Drosophila, and was also found to occur in the ovarian 

 follicle cells, the spermatogonia and some embryonic cells. This 

 is not an occasional phenomenon, but one which belongs to every 



