26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



to their final position in the larval gonad. In this genus, as already 

 stated, the germ cells lie for a time in the amnio-proctodaeal cavity 

 and then pass through the embryonic wall. They then move along on 

 the posterior mesenteron rudiment to the developing gonad. 



Although the germ cells of fleas lack posterior polar granules, they 

 are nevertheless distinguishable from the other early cells of the 

 embryo by their larger size, their more prominent nuclei, and the 

 usual clearness of their cytoplasm (pi. 5, fig. 48; pi. 12, figs. 89, 90). 

 They enter the surface layer during the blastema stage, as described, 

 and with the completion of blastulation are to be found incorporated 

 in the blastoderm at the posterior end of the egg (pi. 5, fig. 48) . With 

 the appearance of the posterior mesenteron rudiment, the germ cells 

 no longer form a part of the blastoderm, but come to lie on the inner 

 surface of the rudiment (pi. 6, fig. 51). The whole egg section from 

 which this enlargement was made is shown in figure 14 (pi. 2). When 

 the posterior portion of the germ band is involuted into the yolk, the 

 germ cells, together with the posterior mesenteron rudiment, are 

 carried along. At this stage they lie on the embryonic tail at a point 

 near which it is continuous with the amnion. From here they pass, 

 at a later stage, into the epineural sinus where they form two aggre- 

 gates. The two groups of germ cells move slowly forward, one on 

 each side of the differentiating body cavity, where they are finally 

 incorporated into the gonads as described in a later section. 



Of the five workers who heretofore have written on the embryology 

 of fleas, only two have observed the germ cells. One was Balbiani 

 (1875) who, at a later date (1882) published an admirable work on 

 the germ cells of Chironomus. In his paper on the development of 

 the cat flea, he describes what he believes to be the germ cells in a 

 stage somewhat later than that in which the embryonic membranes are 

 completed. He says : "The organ of reproduction is already visible in 

 the form of a small mass of clear cells located along the internal aspect 

 of the abdomen immediately below the posterior margin of the 

 vitellus. No envelope or cellular wall surrounds this group of germ 

 cells." The fact that he describes them as clear cells substantiates the 

 identification of the germ cells herein noted as well as the conclusion 

 that the cytoplasm of flea germ cells is lacking in the polar granules 

 common in those of other insects. 



While Balbiani saw nothing of the early stages of the germ cells, 

 it is apparent that Packard (1872) actually observed them at the time 

 of their constriction. According to the terminology of his time he 

 called them "pole cells." Although Metchnikoff (1866) had already 



