22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



his thorough and extensive study of the embryology of the honey bee, 

 makes no mention of the presence of posterior polar granules in the 

 eggs of this insect. He, too, used Bouin's fluid for fixation and iron 

 haematoxylin for staining and found that the yolk spheres stain deeply 

 by this method. As he could hardly have failed to observe a limited 

 posterior polar area of granules had one been present, it may be 

 inferred that the eggs of the honey bee, like those of fleas, lack these 

 particles. 



While Hegner's interpretation of the significance of posterior polar 

 granules has been accepted by numerous workers, other embryologists 

 have opposed his view. Huettner (1923), particularly, has taken this 

 position. In 1901 Noack reached the conclusion that these granules 

 are yolk spheres. Huettner, however, demonstrated that this is not 

 the case, and showed furthermore that they are not mitochondria. He 

 also noticed, as Noack had already pointed out, that all of the germ 

 cells do not receive an equal number of these granules. Therefore, in 

 contrast to the germ cell determinant hypothesis, he believes it 

 "probable that the posterior polar granules may be only by-products of 

 the posterior germ plasm and have nothing to do with the determina- 

 tion of the germ cells." In spite of his suggestion that the term be 

 laid aside, some recent workers continue to use it. Du Bois (1932), 

 Gambrell (1933), and Butt (1934), all write of these granules as 

 germ cell determinants. The fact elucidated above, that the eggs of 

 certain insects such as fleas lack posterior polar granules, appears to 

 be conclusive evidence that these structures are not the differentiators 

 of the primordial germ cells. 



The two general types of germ cell differentiation in insects are 

 illustrated by Miastor and Drosophila respectively. In the former, 

 as shown by the studies of Metchnikofr (1866), Kahle (1908), and 

 Hegner (1912 and 1914a), a single primordial germ cell gives rise 

 to all the germ cells of the organism. In Miastor the parent nucleus 

 of all the germ cells is differentiated from its fellows in the 8 nucleus 

 stage when it migrates into the posterior polar plasm. Thereafter, this 

 entire polar plasm with its nucleus is cut off from the egg to form the 

 primordial germ cell, which by subsequent divisions produces all the 

 gonial cells of this fly. 



In contrast to this single cell origin for all the gonial cells, the 

 majority of insects appear to derive their germ cells by a somewhat 

 different method. In Drosophila, for example, as shown by Huettner 

 (1923), a variable number of cleavage cells migrate into the posterior 

 polar plasm and each is constricted off as a primary germ cell. In 

 Drosophila this number varies from 6 to 20. In some eggs, the total 



