GENITAL ORGANS. 



353 



cells" first shift between the blastoderm-cells (Fig. 174 67), and then 

 into the interior of the embryo, where, in later stages, they become 

 arranged symmetrically into two groups, and, surrounded by cells of 

 the neighbouring tissue, become transformed into the genital rudi- 

 ment (Metschnikoff). 



In Chironomus (Fig. 175), two "pole-cells" become constricted 

 from the posterior pole of the egg almost simultaneously (Balbiani), 

 these, by division, yielding groups of four and eight cells. Just 

 as in Cecidomyia, these cells are taken into the interior of the 

 embryo (Fig. 175 C), and break up into two groups lying at 



Fig. 174. — First ontogenetic stages in the parthenogenetic egg of the Cecidomyia larva 

 (after Metschnikoff). b, peripheral protoplasm ; 6/, blastoderm ; d, central food-yolk ; 

 /, cleavage-nucleus; n, degenerating nutritive cells (the so-called corpus luteum); pz, 

 "pole-cells." 



the sides of the proctodaeal invagination. In quite young larvae 

 that have hatched from the egg, these two spindle-shaped groups, 

 the cells of which soon increase in number, can be recognised at the 

 sides of the dorsal vessel, surrounded by a distinct membrane which, 

 anteriorly and posteriorly, is continued into a ligamentous structure. 

 The anterior corresponds to a terminal filament, the so-called Midler's 

 filament. It is inserted into the dorsal vessel, and was held by 

 Schneider (Xo. 74) to be muscular, this author consequently tracing 

 back the genital rudiment of the Diptera to a transformed fibre of 

 the alary muscle of the heart, an assumption which was refuted by 



2 a 



