NO. 3 EMBRYOLOGY OF FLEAS KESSEL 2."] 



indicated the significance of corresponding cells in Miastor, Packard 

 was content to state that they disintegrated when he lost track of them. 

 His contributions to this subject are best shown by quoting his 

 statements : 



There was observed a vacant space between the yolk and the chorion at the 

 posterior pole, the egg contents completely filling out the opposite end. Also 

 at this time the end of the egg distinctly bulges out, and in this shallow sinus are 

 four distinct polar cells and a small indistinct one in addition, they are dis- 

 tinctly nucleated just as in Chironomus. There seems to be a membrane 



(I suppose the vitelline membrane) retaining these polar cells in place 



About two hours later, the vacant space at the posterior pole of the egg has 

 disappeared, and the yolk and protoblastoderm have pushed up against the 

 vitelline membrane and polar cells. In half an hour's time more, the yolk mass 

 has advanced half way to the polar cells. At this time there were no signs 

 of blastodermic cells. A few hours later, probably not over thirty after the egg 

 had been laid, the blastoderm cells had appeared around the yolk. Soon after 

 this the polar cells break down and disappear. 



Weismann (1863) does not mention the germ cells in the dog flea 

 eggs which he studied, yet he goes into some detail in his discussion of 

 the "pole cells" which he observed in Chironomus and CalUphora, 

 the other two insects considered in the same paper. This is accounted 

 for by the fact that the rather opaque chorion in Ctenocephalldes canis 

 makes it impossible to observe the finer details of structure in whole 

 mounts. As he was unsuccessful in his attempts to remove the chorion 

 without injury to the vitellus and embryo, his discussion of the embry- 

 ology of fleas is very limited. In CalUphora, and Chironomus particu- 

 larly, the chorion is transparent. 



It appears that the germ cells of fleas have not been studied by the 

 section method heretofore, although both Tikhomirowa (1890) and 

 Strindberg (1917) used this technique in their investigations on 

 pulicid embryology. The former does not mention the germ cells, 

 while Strindberg says that they are not disclosed in the eggs of 

 ArcJiacopsylla erinacei. 



FORMATION OF THE GERM BAND 



At the conclusion of blastulation, the blastoderm cells are equally 

 distributed over the surface of the egg. This condition is characteristic 

 of the undifferentiated blastula. Soon, however, there occurs a con- 

 centration of cells toward the ventral midline and polar surfaces of 

 the egg. This crowding of the cells toward the ventral midline is the 

 first step in the formation of the ventral plate which in turn anticipates 

 the germ band (pi. 6, fig. 50). The cells of the median dorsal region 

 appear to be somewhat flattened by the lateral tension exerted upon 



