20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



(1866) found similar pole cells in Miastor and succeeded in tracing 

 them to the larval gonad, thereby indicating their true character. His 

 work was confirmed by later workers, particularly Balbiani (1882) 

 who saw in Chironomus not only the constriction of these cells but 

 their reentry into the interior of the egg and incorporation into the 

 gonads as well. All the germ cell work up to this time was done on 

 entire eggs, and with the introduction of modern sectioning and stain- 

 ing techniques, further evidence was accumulated to show that the 

 pole cells are in reality the primordial germ cells. Ritter (1890) 

 especially, again working on Chironomus, was able to trace, by means 

 of extensive sections, the development of the germ cells up to the 

 time that they reached their final positions in the gonads of the fully 

 formed larva. Ritter's work was reexamined and confirmed in the 

 main by Hasper (191 1). Subsequent to these earlier fundamental 

 studies, germ cells have been discovered in the eggs of a number of 

 other insects. These investigations indicate, however, that the differen- 

 tiation of germ cells among insects takes place by two general methods. 

 Before comparing these two methods it is necessary to discuss the 

 characteristics of the posterior polar plasm and the role it plays in the 

 origin of the primordial germ cells of hexapods. 



Weismann (1863) described what he termed "yolk granules" in 

 the pole cells of Chironomus. Ritter (1890), in his study of the germ 

 cells of this same fly by the section method, demonstrated that Weis- 

 mann's "yolk granules" came from a disk-shaped mass of granular 

 substance located in the periplasm at the posterior pole of the egg. 

 Hasper (191 1), in his reinvestigation of Ritter's work, was able to 

 show that this granular substance was incorporated into the cytoplasm 

 of the germ cells. This incorporation of granular plasm by the germ 

 cells has been reported in numerous insects and has led to the belief 

 which is held by some workers that this substance possesses the peculiar 

 power of incorporating itself into the cytoplasm of differentiating 

 cleavage cells and thereby transforming them into germ cells. Hegner, 

 in a series of publications dating from 1908 to 191 7, champions this 

 view and terms the granules in question "germ cell determinants." 



According to the descriptions of numerous other authors, these 

 posterior polar granules are congregated in a disk-shaped area in the 

 peripheral periplasm at the posterior end of the egg. Gambrell (1933), 

 in discussing this mass of granules in Simuliuni, says that it lies just 

 beneath the surface at the posterior end and appears as a sharply out- 

 lined, irregular, and darkly staining body which is completely imbedded 

 in the formative protoplasm. Butt (1934) describes it for Sciara as 



