NO. 3 ' EMBRYOLOGY OF FLEAS KESSEL 1 3 



polyspermy, but if other spermatozoa have entered this particular egg 

 they have apparently disintegrated. The actual process of fertilization, 

 or the fusion of the male and female gametic nuclei, is shown in 

 figure 37 (pi. 4). It occurs in the central region of the egg, as a rule 

 somewhat toward the anterior pole. The zygotic nucleus resulting 

 from syngamy is shown in figure 38, and the complete section from 

 which this enlargement was made is shown in figure 39. 



CLEAVAGE 



As is typical for the eggs of insects, cleavage in fleas is meroblastic 

 peripheral, owing to the centrolecithal nature of the ovum. By a series 

 of synchronous mitotic divisions, several generations of so-called 

 cleavage cells are produced by the zygotic nucleus. As Blochmann 

 (1887b) first pointed out, these bodies are not independent cells, 

 inasmuch as each is connected by protoplasmic strands to the proto- 

 plasmic reticulum which ramifies throughout the vitellus, and by this 

 reticulum, in turn, to the periplasm. In flea eggs, each cleavage nucleus 

 is surrounded by a star-shaped protoplasmic mass, and this is con- 

 tinuous with the protoplasmic reticulum. Such cleavage nuclei, there- 

 fore, have the same general appearance as the zygotic nucleus. Like it, 

 they stain rather faintly. 



Synchronous cleavage divisions have been reported for a number 

 of insect species. Huettner ( 1923) found them to occur in Drosophila, 

 Auten (1934) in Phormia, and Butt (1936) in Brachyrhinus. Their 

 presence in flea development, however, has not been observed hereto- 

 fore. 



During the early cleavage divisions the resulting nuclei lie rather 

 close together in the central region of the egg. Not until after the 

 completion of the fifth such division, when the egg is in the 32 

 nucleus stage, is there an indication of any orderly migration of 

 the cleavage nuclei toward the periphery. Following the next or sixth 

 cleavage division, when 64 segmentation nuclei are present in the 

 egg, the majority of these have definitely begun their migration to 

 the surface. In doing this they collectively assume a hollow sphe- 

 roidal arrangement which is proportionate in all its dimensions to the 

 outline of the egg. The nuclei which outline this spheroid are all 

 approximately equidistant from the cortical ooplasm which forms the 

 surface of the egg mass. Huettner (1923) says that in Drosophila 

 the cleavage nuclei start their migration to the surface after the eighth 

 cleavage division, or when the egg is in the 256 nucleus stage. In flea 

 eggs, therefore, this migration starts somewhat earlier. 



