351 



IN SECT A. 





Balbiani. The posterior terminal filament is the rudiment of the 

 paired efferent ducts of the genital gland. The division of the 

 inner cells of the ovarian rudiment gives rise to rosette-like groups 

 of cells, each of which corresponds to an ovarian tube. Eichter's 

 more recent accounts (No. 71) agree with the above statements of 

 Balbiani. 



In the Aphidae, just as in the Diptera, the first rudiment of the 

 genital organs becomes distinct at a very early stage. Even in those 

 early stages in which the first rudiment of the amniotic cavity forms 

 by invagination at the posterior pole (p. 279), and before the 

 formation of the lower layer, a cell-group (the genital rudiment) 

 severs itself from the wall of this invagination, and, as a paired 



rounded mass, comes to 



B fe^D^^o^j lie in tne inside of tne 



embryo. This cell-group, 

 according to Balbiani 

 and "Witlaczil, is de- 

 rived by division from 

 a single cell. It becomes 

 horseshoe - shaped, and 

 breaks up into a number 

 of rounded cell-accumu- 

 lations, which become 

 arranged in equal num- 

 bers on each side of the 

 median plane of the body 

 and represent the rudi- 

 ments of the terminal 

 chambers of the ovarian 

 tubes. These cell-masses 

 are enveloped in an 

 epithelial cover which, 

 anteriorly, passes into 

 the terminal filaments and, posteriorly, into the efferent ducts. The 

 origin of this epithelial covering is doubtful. The efferent ducts of 

 the different ovarian tubes fuse on each side to form a common 

 oviduct, and this is continued into an unpaired ectodermal invagi- 

 nation lying beneath the proctodaeum ; this invagination gives 

 rise to the accessory genital organs (Metschnikoff, Witlaczil, 

 Will). 



O 2°o° ( )o°° a 









. VAfro ° * 



°OoO g ' 



Fir.. 175. — Three longitudinal sections through Chironomus 

 embryos (after Ritter). In A, the "pole-cells" (pz) 

 lie outside the developing blastoderm. In B, the 

 pole-cells have pressed in between the blastoderm-cells. 

 In C, they lie within the embryo, b, peripheral proto- 

 plasm ; bl, blastoderm ; d, food-yolk ; k, nuclei of the 

 blastomeres ; p, "pole-cells.'' 



