NO. 3 EMBRYOLOGY OF FLEAS KESSEL 1 7 



the absolute centers of their forming, respective cell territories. In- 

 stead, each is located somewhat toward the outer margin. With the 

 completion of the lateral cell walls, the cell territories still remain 

 incompletely demarcated, as the cytoplasm of their bases is still con- 

 nected. It is also continuous with the reticular protoplasm of the 

 interior of the egg. Although these phases of blastulation are typical 

 for centrolecithal insect eggs, the final step, consisting of the develop- 

 ment of the inner walls of the blastoderm cells, varies as to details in 

 different forms. In the eggs of certain insects such as Hydrophilus 

 (Heider, 1889) and Apis (Nelson, 1915) an inner peripheral or 

 cortical protoplasmic layer is produced. It is believed to be composed 

 entirely of protoplasm which has come to the surface with the cleavage 

 nuclei. This zone has only a temporary existence owing to the fact 

 that it is ultimately absorbed by the blastoderm cells. A true layer 

 of inner peripheral protoplasm was not found by Marshall and Dernehl 

 (1905) in their study of Polistcs, although a small amount of cyto- 

 plasmic substance was observed just inside the bases of some of the 

 completed blastodermic cells. An inner layer of peripheral protoplasm 

 is said by Carriere and Burger (1897) to be entirely lacking in 

 Chalk odoma and Anthophora. As the eggs of these last two genera 

 have a relatively thin periplasmic layer, and as this is also the case in 

 flea eggs, an inner peripheral protoplasmic layer was expected to be 

 lacking also in the developing ova of these insects. A deficiency of 

 cytoplasmic material in the cortical layer would be expected to neces- 

 sitate the incorporation at once into the blastodermic cells of all the 

 protoplasm which had come to the surface with the cleavage nuclei. 

 However, careful examination of sections of flea eggs representing 

 this stage indicates an agreement with the eggs of Polistcs rather than 

 with those of Chalicodoma and Anthophora. Although thin, irregular, 

 and ill-defined, cytoplasmic patches may be seen lying just below the 

 bases of certain of the blastoderm cells. This may be regarded as 

 homologous to the definite inner layer of peripheral protoplasm of 

 certain other insects. 



TROPHONUCLEI 



As already stated, after the sixth cleavage division when 64 seg- 

 mentation nuclei are present in the egg, a variable number of these 

 start migrating toward the surface as the preblastema nuclei. Within 

 the spheroidal figure which they outline are the remaining nuclei 

 which, with their progeny, stay in the central portion of the egg. Here 

 they probably serve to liquefy the yolk which surrounds them, making 



