SIPHONAPTERA AND DIPTERA 361 



Hatching occurs after six days of development under controlled 

 temperature and humidity conditions. After swallowing the amniotic 

 fluid, the young larva escapes from the egg through a slit in the shell. 



The type of development of the hedgehog flea resembles that of other 

 fleas although Strindberg (1917) failed to find the early development of 

 the germ cells at the posterior end of the egg. It is possible that Strind- 

 berg overlooked the germ cells, since Kessel found them in the species 

 studied by him; and Packard (1872) states that at a very early stage 

 before the completion of the blastoderm, four distinct "polar cells" 

 (germ cells) are found at the posterior pole of the egg of Pulex canis 

 (Ctenocephalides canis) . Likewise in the plague flea (Xenopsylla cheopis) 

 germ cells are clearly evident before the completion of the blastoderm, 

 as shown by the work of Cameron (1940) whose slides also distinctly 

 show the presence of yolk cells at this stage. Balbiani (1875) found a 

 group of micropylar openings at each end of the egg of P. felis, a condi- 

 tion that Cameron also found in X. 



DIPTERA 



The Mourning Gnat {Sciara coprophila) 



At an early age the nucleus is at the center of the fertilized egg lying 

 in the yolk. A layer of cytoplasm, the periplasm, which is much thicker 

 at the poles, surrounds the deutoplasm. At the posterior pole in the 

 periplasm lies a saucer-shaped granular plate, the oosome, or germinal 

 cytoplasm. 



The fusion nucleus is no sooner formed after the union of the male 

 and female pronuclei than it divides. The resultant daughter nuclei 

 move apart, and a second division takes place, four nuclei being formed. 

 As this process continues regularly through four divisions, or until there 

 are 16 nuclei in all, each nucleus surrounded by a mass of cytoplasm 

 moves toward the periphery of the egg (Fig. 313). 



At the fifth division, according to Du Bois, two of the cleavage nuclei 

 penetrate the oosome and become differentiated as the germ cells. They 

 apparently absorb the granular material of the disk (Fig. 315) and become 

 greatly enlarged and altered in appearance. They continue their migra- 

 tion through the periplasm that surrounds the oosome until they lie 

 outside in contact with the vitelline membrane where they divide until 

 there are from 22 to 28, forming a lumpy protuberance at the posterior 

 end of the egg. 



While the two nuclei are passing through the oosome, the other nuclei 

 in migrating toward the periphery continue to multiply, arranging them- 

 selves in the periplasm in a single layer forming a blastoderm around the 

 yolk (Figs. 313, 314). Cell walls form around them on the outside, but 



