NO. 3 EMBRYOLOGY OF FLEAS KESSEL 1 9 



nuclei into the vitellus from both the serosa and the ventral plate. 

 Graber (1871) had previously shown that in forms possessing primary 

 trophonuclei, these are augmented by additional or secondary vitello- 

 phags which enter the yolk from the germ band. Numerous other 

 workers, including Korotneff (1885) for Gryllotalpa, Noack (1901) 

 for CalUphora, Nelson (191 5) for Apis, Butt (1936) for Brachy- 

 rJiinus, and Lassmann (1936) for Melophagus, have substantiated his 

 observations. In flea eggs, such an augmentation of the primary vitel- 

 lophags by others of a secondary nature occurs, notwithstanding the 

 statement of Tikhomirowa (1892) to the contrary. This augmentation 

 is by immigration from the surface. These secondary trophonuclei 

 may enter the yolk either from the blastoderm or from the mesoderm 

 when this germ layer has differentiated. None of them appear to come 

 from the serosa as Patten (1884) described for Ncophylax. In spite 

 of their origin from the completely delimited cells of the blastoderm 

 or mesoderm, the secondary vitellophags appear to lose their cellular 

 nature as they enter the yolk. Their cell boundaries break down and 

 they become incorporated into the cytoplasmic syncytium which rami- 

 fies through the vitellus. Having lost their demarcations, these vitel- 

 lophags can no longer be regarded as independent cells. Instead, they 

 must be termed trophonuclei like the primary vitellophags. Such 

 secondary trophonuclei appear indistinguishable from those of the 

 primary group once they have passed deeply into the yolk. When first 

 given off by the blastoderm, they stain rather deeply and correspond 

 in the density of their coloration to the cells of the superficial layers. 

 The subsequent decrease in the staining ability of the secondary 

 trophonuclei appears to be due to the dispersion or dissolution of their 

 substance. A similar phenomenon has been observed by Heymons 

 (1895) in Gryllotalpa, Gryllus, and Forficula, by Friederichs (1906) 

 in Mcloc, and more recently by Eastham (1927) in Pieris. 



ORIGIN OF THE GERM CELLS 



The origin of the primordial germ cells in insects was first observed 

 by Robin (1862) in Chironomits. He failed, however, to recognize 

 their true nature for he regarded them as polar bodies. Weismann 

 (1863), again in Chiroiwmus and also in CalUphora, observed not 

 only the separation of these cells from the posterior pole of the egg- 

 but also found that they reentered the egg and finally disappeared 

 among the embryonic tissues. He concluded, therefore, that they were 

 not polar bodies and termed them "pole cells" instead. Like Robin, 

 however, he failed to understand their significance. Metchnikoff 



