368 A^. M. Stevens 



prereductional pairing of homologous chromosomes was first 

 noticed in the oogonia and ovarian folhcle cells of Drosophila. 

 An attempt was made to ascertain whether such a pairing occurs 

 in embryonic cells. Very little evidence was obtained. In the 

 prophase of one mitosis paired chromosomes were found (Fig. 

 83). Fig. 84 is the equatorial plate of a segmentation stage. In 

 both cases the pairs appeared to be equal. 



g Eristahs tenax 



A considerable number of these flies were captured on some 

 late blooming mustard plants in October. The material was in 

 exceptionally favorable condition, and a complete series of draw- 

 ings was obtained. The outer wall, or capsule, of the testis was 

 thinner and more permeable to fixing fluids than in most of the 

 other species studied and it was therefore possible to work with 

 both sections and aceto-carmine preparations. This fly belongs 

 to the family Syrphidae, but the chromosomes in most respects 

 resemble those of the Muscidae. The heterochromosome bivalent 

 is diff^erent in form from that of any of the Muscidae described 

 above; it however consists of a larger and a smaller component 

 united in a somewhat diff'erent way from the corresponding ele- 

 ments in Drosophila. 



Among the spermatocytes, several follicle cells were found in 

 mitosis; the chromosomes of one such is shown in Fig. 85. The 

 spermatogonia! chromosomes are paired in prophase but sepa- 

 rate and form a flat plate in the metaphase as seen in Fig. 86^ 

 where the two heterochromosomes (A^, h^ are conspicuously 

 unequal in size. In this form there is a distinct synizesis stage, 

 as shown in Fig. 87, from a section of material fixed with Gilson's 

 mercuro-nitric fluid and stained with thionin. The cysts in which 

 this stage occurs border upon the spermatogonia! region of the 

 testis. The outline of the chromosomes is visible and in the 

 next stage the chromosomes are distinctly bivalents. Later they 

 become more diff'use, but do not appear to form an even spireme 

 at any stage. Fig. 89 is a growth stage, showing the heterochro- 

 mosome group (A), a pair of m-chromosomes and the other chro- 



