362 N. M. Stevens 



the male and an equal pair of large heterochromosomes in the 

 female. An egg which is fertilized by a spermatozoon containing 

 the smaller heterochromosome produces a male, while one which 

 unites with a spermatozoon containing the larger heterochromo- 

 some produces a female. 



Although there is no distinct synapsis stage visible in the devel- 

 opment of the spermatocytes of Musca domestica, the method of 

 synapsis is without doubt indicated by the side-to-side pairing of 

 chromosomes of equal length in the prophases of both spermato- 

 gonial and oogonial mitoses. The final synapsis is a closer union 

 of the homologous chromosomes, and the first spermatocyte 

 division separates the members of each pair instead of dividing 

 each chromosome as in the spermatogonia. 



2 Calliphora vornitoria 



The chromosomes in this species are similar to those in Musca 

 domestica. Both members of the unequal pair of heterochromo- 

 somes are smaller, as may be seen in a spermatogonial metaphase 

 (Fig. 10). Pairing of homologous chromosomes is also evident 

 here. In the growth stage (Fig. 11) the heterochromosomes are 

 associated with a plasmosome as in many species of Coleoptera. 

 Two views of the metaphase of the first spermatocyte mitosis are 

 shown in Figs. 12 and 13, and an anaphase in Fig. 14. Two 

 metaphases and an anaphase of the second division appear in 

 Figs. 15, 16 and 17. The equal pair of heterochromosomes in 

 the female is clearly shown in two oogonial metaphases (Figs. 

 18 and 19). In this case we have further evidence of the side-to- 

 side pairing of homologous chromosomes in the spermatogonia 

 and oogonia. 



J Liicilia ccesar 



Only a few specimens of this species were captured and the 

 series of stages is incomplete. No spermatogonial or oogonial 

 metaphases were found. In the growth stage a pair of m-chro- 

 mosomes is present with an enormous heterochromosome bivalent 



