DEVELOPMENT OF THE MALE GLANDS 



581 



two spindle-shaped groups, whose cells soon increase in number, may be seen 

 situated dorsally on the side of the heart, enveloped by a clear cellular mem- 

 brane which ends before and behind in a ligament-like terminal thread. The 

 anterior terminal thread is the rudiment of the so-called Miiller's thread. The 

 thread at the posterior end is the rudiment of the paired efferent passage of 

 the genital glands. Through a division of the cells lying in the interior of the 

 rudiments of the ovaries, there results the formation of a rosette-shaped group 

 of cells which corresponds to the contents of an ovarian tube. With this view 

 of Balbiani the later observations of Ritter agree. 



As in the Diptera, so in the Aphides, the first germs of the genital organs are 

 differentiated very early in life. In the early stage in which through an in vagi- 

 nation from the hinder pole of the egg the first rudiment of the amnion-cavity 

 is formed, a group of cells becomes separated from the wall of this invagi- 



FIG. 551. First developmental stages of tin- parthenogenatic eggs of the larva of Cecidomvia : 

 6, peripheral protoplasmic layer (Keimhautblastem) ; !>!, blastoderm; d, central yolk; /, division- 

 nuclei; 11, nutritive cell (' corpus luteum") about to break up; pz, polar cells. After Metschni- 

 koff, from Korschelt and Heider. 



nation before the formation of the lower layer, which at this time lies as an 

 unpaired roundish mass within the embryo. This group of cells, according to 

 Balbiani and Witlaczil, has arisen by division of a single cell. Afterwards it 

 becomes horseshoe-shaped and divides into a number of roundish masses of 

 cells, which are arranged in similar numbers on each side of the median plane 

 of the body, and form the rudiments of the terminal fan (Endfacher). They 

 are covered by an epithelial envelope which passes anteriorly into the terminal 

 threads, posteriorly into the efferent passage. The origin of this epithelial case 

 is unknown. The efferent passages of the separate ovarian tubes are united into 

 a common oviduct, and this fuses with an unpaired ectodermal invagination 

 lying under the hind intestine from which the accessory sexual organs are 

 formed. (Korschelt and Heider from Metschnikoff, Witlaczil, Will.) 



In the Hymenoptera Ganin has observed in the embryo of Platygaster the 

 rudiments of the sexual glands in the form of two rounded masses situated near 



