24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



the only exception being that in some cases the most posterior nuclei 

 arrive at the periphery shortly after the others. Judging from their 

 position, these extreme posterior polar nuclei are to be regarded as 

 future germ cell nuclei. Their delayed arrival at the periplasm appears 

 to be at variance with the case of Drosophila, for which Huettner 

 (1923) says that the polar nuclei apparently begin their migration a 

 trifle earlier than the preblastema nuclei. As already suggested, the 

 retarded arrival of the posterior nuclei in flea eggs may be ascribed to 

 the fact that in certain cases the fusion nucleus is located somewhat 

 anterior to the center of the egg when it divides. 



The first appearance of protoplasmic pockets anticipating the con- 

 striction of germ cells, occurs soon after the nuclei have entered the 

 periplasm to form the blastema. This is at the 128 nucleus stage, or 

 following the seventh cleavage division. At this stage, some 15 nuclei 

 are present in the periplasm in a sagittal section. The beginning of 

 germ cell protrusion in flea eggs is relatively precocious when com- 

 pared with Drosophila. In fact, the migration of the preblastema and 

 pregerm cell nuclei is likewise earlier for the fleas than for the 

 pomace fly. In the latter, according to Huettner (1923), it is not until 

 after the next or eighth cleavage division that the nuclei begin to 

 migrate toward the periphery. 



The sections prepared for this study show that as the protrusions 

 caused by the first germ cells become more prominent, others which 

 are more lateral in position begin to make their appearance. Even 

 when the most posterior cells are completely constricted, others may 

 be just beginning to bulge out. There is consequently such a gradual 

 transition from the ordinary blastema nuclei to the more median germ 

 cells that it is impossible to determine, with any degree of accuracy, 

 a boundary of distinction between the two types of nuclei. The first 

 germ cells to be constricted appear to become complete during the 

 second blastema substage, or after the eighth cleavage division. Con- 

 striction of those more lateral in position may continue into the early 

 part of the third blastema substage following the subsequent or ninth 

 cleavage division. The germ cells then reenter the nucleated periplasm 

 during either the latter part of the third or during the fourth blastema 

 substage before the cell territories of the blastoderm are delimited. 

 Figure 48 (pi. 5) shows the germ cells after their return into the body 

 of the egg. This reincorporation of the germ cells in the blastema 

 surface is in accordance with the recent works of Butt (1936) on 

 Brachyrhinus, and of Wray (1937) on Calendra, but at variance with 

 the account of Hegner (1909a) for chrysomelid beetles. Hegner 

 states that the germ cells of these insects remain outside the egg body 



