CHAPTER XX 



SIPHONAPTERA AND DIPTERA 



SIPHONAPTERA 



The Fleas 



Except for a few brief papers dealing with the embryology of the 

 fleas of the dog, cat, and hedgehog but little work had been done on this 

 group of insects until the appearance of the work of Kessel (1939) which 

 is an extended account of the development of the cat flea (Ctenocephalides 

 felis), the rat flea (Nosopsyllus fasciatus), and the wood-rat flea {Hystri- 

 chopsylla dippiei). The following abstract, based upon Kessel's work, 

 appUes to the three species just mentioned unless otherwise stated. 



The eggs of these fleas are regularly prolate-spheroidal in shape and 

 when first deposited are glistening white. The sculpturing of the chorion 

 is least distinct on that of the cat flea. Micropylar openings are found 

 on both poles of the egg. Although the large number of micropylar 

 openings suggests polyspermy, it was not observed by Kessel. 



Early cleavage stages are synchronous. After the sixth division the 

 cleavage cells definitely begin their migration to the egg surface. Rela- 

 tively few cleavage cells remain behind in the yolk as vitellophags, or 

 trophonuclei. After the seventh cleavage, or the 128-cell stage, the 

 nuclei reach the periplasm, the mitotic spindles in this division being 

 parallel to the egg surface. Further repeated divisions of the cells in 

 th*e periphery result in the completion of the blastoderm. In flea eggs 

 the primary vitellophags (trophonuclei) are augumented by the immigra- 

 tion of cells (secondary trophonuclei) from the periphery back into the 

 yolk, where they lose their cell boundaries and become incorporated into 

 the cytoplasmic syncytium which ramifies through the yolk and where 

 they appear indistinguishable from those of the primary group. 



In the fleas a variable number of cleavage nuclei pass into the posterior 

 periplasm and constrict off from the body of the egg surrounded by 

 cytoplasm, to become the first germ cells. This occurs following the 

 seventh cleavage stage. Before the walls of the blastoderm cells appear, 

 the germ cells migrate back into the body of the egg. The oosome, or 

 the so-caUed ''germ-line determinant," is lacking in the eggs of the 

 fleas. Nevertheless the germ cells are distinguishable from other early 

 cells of the embryo by their larger size, their more prominent nuclei, and 

 the usual clearness of their cytoplasm. With the appearance of the 



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