NO. 3 EMBRYOLOGY OF FLEAS KESSEL J 



slide, all traces of water were quickly wiped from the under surface 

 of the glass, and the slide was passed directly into xylol. Absolute 

 tertiary butyl alcohol was used following the removal of the paraffin 

 by xylol because it was desired to keep the celloidin matrix intact, and 

 this alcohol does not dissolve celloidin. 



In order to make certain that the sections would remain firmly 

 attached to the slide during the staining process, a step was adapted 

 from Galigher (1934) and introduced. into the procedure at this point. 

 This consisted of dipping the slide into a 1 percent solution of ether- 

 alcohol celloidin and then passing it into chloroform to harden the 

 film. Thereafter, the slide was transferred to 95 percent tertiary 

 butyl alcohol for the beginning of hydration preparatory to aqueous 

 staining. The lower alcohols used in the series were either tertiary 

 butyl or ethyl solutions, inasmuch as the latter possessed sufficient 

 water to prevent the dissolution of the celloidin matrix and protective 

 film. It was necessary that dehydration following staining be com- 

 pleted with tertiary butyl alcohols. 



Sections were stained by various methods, the combination of 

 Delafield's haematoxylin with Eosin Y as a counterstain being used 

 for most of them. Mayer's haemalum with a counterstain of Fast 

 Green FCF, and Heidenhain's iron haematoxylin with a counterstain 

 of Eosin Y were also used to some extent. Heidenhain's iron haema- 

 toxylin was found unsatisfactory for the earlier stages because it 

 colored the numerous yolk spheres so darkly that they dominated the 

 entire picture and obscured both the cleavage nuclei and the tropho- 

 nuclei (pi. 1, fig. 13). 



Because the eggs used in this study were obtainable in such large 

 numbers, it was possible to infiltrate and imbed them en masse, 

 several hundred to a block. No attempt was made at orientation, and 

 those sections which were cut at undesirable planes were ignored 

 (pi. 4, figs. 39, 40). 



CHARACTERISTICS OF THE EGGS 



The eggs of fleas are regularly prolate-spheroidal in shape. Conse- 

 quently, they do not possess any dorsoventral differentiation such as 

 is exhibited by the ova of many other insects. When first deposited, 

 the eggs of all three species of fleas considered in this paper are 

 glistening white. There is a pronounced difference in the degree of 

 their transparency, however. Those of Ctenocephalides felis are rela- 

 tively transparent, those of Hystrichopsylla dippiei less so ; those of 

 Nosopsyllus fasciatus are the most opaque. While the eggs of all fleas 



