NO. 3 EMBRYOLOGY OF FLEAS KESSEL 25 



until blastulation is completed. Their entry later is by means of 

 incompleted points of the blastoderm, termed pole canals. The situa- 

 tion in flea eggs differs likewise from that which Lassmann (1936) has 

 found in Melophagus. In this form, the germ cells do not reenter 

 the egg until much later. Instead, they lie for a time in the differen- 

 tiating amnio-proctodaeal cavity and not until this has deepened con- 

 siderably do they effect an entrance into the embryo. 



By counting the total number of germ cells shown in the series from 

 which the section for figure 46 (pi. 5) was taken, there were found 

 to be eight. As a single nucleus, by three consecutive divisions pro- 

 duces eight nuclei, it might be inferred that these germ cells had a 

 mononuclear origin. Such a circumstance could be harmonized with 

 what occurs in such insects as Miastor or Chironomus. In the latter 

 fly, according to Hasper (1911), a single primordial germ cell protru- 

 sion takes place. This divides into two cells as it constricts from the 

 egg, and this division is followed by two others, so that, in all, eight 

 germ cells are produced. If there were always eight such cells formed 

 in flea eggs it might be assumed that they had originated from a single 

 cleavage nucleus which could not be distinguished from the other 

 nuclei in the egg, owing to the absence of posterior polar granules. 

 An examination of other serial sections of eggs of the same stage, 

 however, shows that in fleas the total number of germ cells produced 

 is variable. Some eggs possess as few as 5, whereas others have as 

 many as 12. Reference has already been made to the synchronous 

 division of the cleavage and blastema nuclei. Because of the fact that 

 the germ cells begin to push out as soon as the immigrating nuclei have 

 entered the periplasm, it is apparent that all the germ nuclei were 

 produced by the seventh cleavage. Obviously, therefore, they origi- 

 nated simultaneously with all of the other nuclei present in the egg. 

 It is consequently impossible for any total number (also total progeny) 

 of germ cells numbering between 5 and 15, other than 8 (2 3 ) to be 

 produced from a single ancestral nucleus. The presence of variable 

 numbers of germ cells in the eggs of fleas indicates for them a poly- 

 nuclear origin. 



The further history of germ cells in insects has until now been 

 traced completely only in forms possessing differentiating posterior 

 polar granules. For example, Hegner (1909a) found, for Calligrapha, 

 that after reentering the eggs by the pole canals, these cells creep along 

 between the yolk and the germ band to form, eventually, two groups 

 near the developing coelomic sacs. These aggregates acquire follicular 

 envelopes, probably mesodermal, and the gonads are completed. Like- 

 wise Lassmann ( 1936) was able to trace the germ cells of Melophagus 



