112 GEOEGO ORIHAY SHINJI 



9. THE ORIGIN OF THE GERM CELLS 



In coccids the germ cells first become noticeable simultaneously 

 with the invagination of the germ band at the posterior end of 

 the egg. They are characterized by their large size, oval shape, 

 and the clearer nuclear appearance as compared with the other 

 cells, and, above all, by the feeble staining reaction. Increasing 

 in numbers by mitotic division, they remain for a time at the 

 point where the invagination of the germ band occurs. The 

 earliest stage in the history of these cells which has been observed 

 is shown in figure 81. 



At the time the invagination becomes clearly visible, the germ 

 cells begin to migrate, one after another, toward the anterior 

 end of the egg where the colony of symbiotic organisms is located. 

 As soon, however, as they reach the mass of organisms, some of 

 them become actually imbedded between the spores of the 

 organisms, others elongate to form a sheath around it, while the 

 remaining cells aggregate superficially around the colony of 

 organisms (figs. 90, 92, and 133). Those cells that become 

 imbedded in or encircle the organisms remain in that condition 

 throughout the rest of the embryonic and larval life and con- 

 stitute the so-called secondary yolk cells of Will ('88) (fig. 90). 



As already mentioned, the elongation of the embryo continues 

 beyond the anterior end of the egg so that the posterior portion 

 gradually curls over the thoracic region, but the germ cells and 

 the colony of the symbiotic organisms remain stationary. The 

 region where these cells, for the second time, become localized, 

 corresponds with the third and the fourth abdominal segments 

 of the larvae as well as of adult female. With the segmentation 

 of the abdominal region of the embryo, the germ cells become 

 divided into the left and right halves. 



With the revolution and subsequent shortening of the embryo 

 an invagination appears at the ventral surface of the ninth 

 abdominal segment. The germ cells, which have for a long time 

 remained stationary around the symbiotic organisms, then 

 migrate toward and finally settle at the apex of the vaginal invag- 

 ination (fig. 119). 



