NO. 3 EMBRYOLOGY OF FLEAS KESSEL 21 



follows : "At the posterior pole in the periplasm lies a saucer-shaped 

 granular plate, the germinal cytoplasm or germ line determinant." 

 Huettner (1923), in his work on Drosophila, used Heidenhain's iron 

 haematoxylin almost exclusively for staining his sections and found 

 that the posterior polar granules stained black like the yolk spheres. 

 Likewise Auten (1934), working on Phormia, found that the posterior 

 granular plate stained deeply with Heidenhain's iron haematoxylin. In 

 view of these and other similar observations which appear in the litera- 

 ture, it was expected that a typical disk-shaped area of darkly staining 

 granules would be found at the posterior pole of the flea egg. However, 

 in none of the numerous preparations of pulicid eggs made during this 

 investigation is such an area indicated. The general appearance of the 

 posterior polar plasm of flea eggs has been described in a previous 

 section. In view of the importance of this substance in the differentia- 

 tion of the germ cells, the following further details are given. At the 

 posterior pole the periplasm is shown to be widened, in a few sections, 

 to form more or less of a cap (pi. 5, fig. 43). This cap is entirely free 

 from yolk spheres. This is its chief distinguishing characteristic, for, 

 in the remaining portions of the periplasm, the yolk structures often 

 appear to extend nearly to the surface. This posterior polar cap is 

 uniformly faintly granular like the rest of the peripheral protoplasm. 

 Furthermore, like the other regions of the egg, it may possess Bloch- 

 mann's corpuscles. But never in any of the sections prepared for this 

 study does it contain any structures which may be interpreted as 

 corresponding to the darkly staining polar granules of other insect 

 eggs. As previously stated, some of the sections were stained with 

 Heidenhain's haematoxylin and, although the yolk spheres were 

 colored black, there were no similarly stained posterior polar granules 

 present. The remainder of the sections made of the early stages were 

 stained with Delafield's haematoxylin with a counterstain of eosin. 

 Gambrell (1933) used this same combination extensively and found 

 the polar granules clearly indicated thereby. Butt (1934) used Bouin's 

 fluid for fixation, a reagent utilized for the majority of the eggs 

 sectioned in this study, and successfully demonstrated the granular 

 plate in Sciara. It may be assumed, therefore, that the fixation and 

 staining techniques employed during the present investigation are 

 favorable to the visibility of posterior polar granules, and their 

 apparent absence in this case may be taken as an indication that such 

 granules do not occur in the eggs of the three flea species under 

 consideration. 



This apparent absence of posterior polar granules, however, appears 

 to be not without precedent in the eggs of insects. Nelson (1915), in 



