114 GEORGO ORIHAY SHINJI 



which he called the pseudovitellus may act as the Keimbahn 

 determinant in coccids and especially in Psylla. 



Following this stage, the germ cells become pushed anteriorly 

 by the invagination of the ventral plate. When, however, the 

 colony of symbiotic organism becomes stationary near the pos- 

 terior end of the egg, the germ cells become also fixed, so to speak, 

 at the place which corresponds approximately to the third and 

 fourth abdominal segments. "When the abdomen begins to show 

 segmentation, the mass of germ cells becomes detached from the 

 colony of parasitic organisms and aggregates to form left and right 

 syncytia, the miniature yet definitive ovaries. 



Thus the chief difference between the germ cells of the mealy 

 bug and of Icerya is that, in the former, the germ cells early 

 migrate to the anterior colony of symbiotic organisms on their 

 own accord, whereas in the latter they move anteriorly, not by 

 their own effort, but by the ingrowth of the germ band. 



The migration of the germ cells, singly or as a group, as I have 

 observed in coccids, is by no means peculiar to this family of 

 insects. Such an instance has already been described for the 

 embryo of the potato beetle, Leptinotarsa decemlineata, by Heg- 

 ner ('14), who found that the germ cells which apparently rest in 

 the amniotic cavity migrated later on into the embryo through a 

 sort of canal at the bottom of a groove in the germ band. 



In the two families of Homoptera, Aphididae and Psyllidae, 

 which are closely related to Coccidae in form, life history, and 

 habits, the primitive germ cells were observed, just as in the case 

 of the coccids above mentioned, early in the embryonic devel- 

 opment. Mecznikow ('66) described the primitive germ cells of 

 parthenogenetically developing embryos of Aphis (Macrosiphum) 

 rosae before the appearance of mesoderm cells. Eighteen years 

 later, Witlaczil ('84) not only confirmed Mecznikow's observation, 

 but was also able to see a single large clear cell, the primitive 

 germ cell, near the blastopore. 



The primitive germ glands of chrysomelid beetles have also 

 been found to appear at about the same stage of development 

 as in the case of aphids. In Clytra laeviuscula, Gastrophysa 

 raphani, Chrysomela menthantri, Lina populi, and Lina tremulae, 



