NO. 3 EMBRYOLOGY OF FLEAS KESSEL 23 



number extruded indicates beyond question that they could not all 

 have arisen by the synchronous division of a single primordial germ 

 nucleus. Noack (1901) found the same type of germ cell differentia- 

 tion occurring in Calliphora. In this form 15 to 20 polar nuclei are 

 pushed out posteriorly at one time. Recently, Lassmann (1936) has 

 shown that germ cell differentiation in Melophagus is of this type. 



In the case of flea eggs, the germ cells develop by a method com- 

 parable to that which takes place in Drosophila, Calliphora, and Melo- 

 phagus. A variable number of cleavage nuclei pass into the posterior 

 polar plasm and constrict off from the body of the egg, surrounded 

 by some of this differentiating ooplasm, to become the first germ cells. 

 Figures 46 and 47 (pi. 5) show several of these completed germ cells 

 lying just within the vitelline membrane. The enlargement for figure 

 46, showing three germ cells, was made from the whole egg section 

 shown in figure 42 (pi. 4) . 



Numerous insect embryologists, including Gatenby (1918) working 

 on Trichogramma, have shown that the polar nuclei which are destined 

 to become germ cell nuclei can in no way be distinguished from the 

 other cleavage nuclei except by their location in the posterior polar 

 plasm. In the eggs of fleas, all the nuclei present in the egg at this 

 stage are similarly indistinguishable, and because the posterior polar 

 plasm is not demarcated, as in most other insect eggs, by its unique 

 granular appearance, it is impossible to recognize them with certainty. 

 Until they begin to protrude from the egg's surface, one may only 

 assume that the nuclei located nearest to the longitudinal axis of the 

 egg at its posterior pole are destined to become germ nuclei (pi. 5, 

 fig. 44). 



Several germ cells make their appearance in the eggs of fleas simul- 

 taneously. The first indication of their constriction is the formation 

 of rounded swellings of the egg surface at the extreme posterior pole. 

 Figure 45 (pi. 5) shows three germ cells bulging out preparatory to 

 constriction. In some eggs, such as the one illustrated, the cytoplasm 

 of the protruding cells may possess a granular appearance. The 

 minute structures which are responsible for this appearance are not to 

 be confused, however, with the described posterior polar granules of 

 other insects. Instead, as already stated, they are to be identified as 

 comparable to Blochmann's corpuscles. Their number is negligible in 

 some preparations and profuse in others. Furthermore, unlike polar 

 granules, they are distributed throughout the entire vitelline and 

 periplasmic regions of the egg. 



In the discussion on cleavage it was stated that in the eggs of fleas 

 all nuclei entering the periplasm usually arrive there at the same time, 



