NO. 3 EMBRYOLOGY OF FLEAS KESSEL I 5 



The number of cleavage nuclei arriving in the cortical ooplasm of 

 flea eggs is inadequate to constitute at once a continuous epithelium 

 such as is characteristic of the blastula stage. As stated above, it is 

 at the 128 nucleus stage or after the seventh cleavage division that 

 the periplasm is first supplied with nuclei. The 15 or so nuclei which 

 are to be seen at the periphery in a median longitudinal section at this 

 stage are rather regularly distributed but widely separated from each 

 other (pi. 1, fig. 4). This condition contrasts with the situation which 

 occurs in Calliphora (Blochmann, 1887b) and Hydrophihis (Heider, 

 1889), in which the nuclei reach the periplasm closely crowded to- 

 gether. In the embryological development of fleas, therefore, there 

 must be a further multiplication of the nuclei in the periplasm before 

 the stage comparable to that in which the nuclei arrive at the periphery 

 in Calliphora and Hydrophilus is reached. For the sake of conve- 

 nience, although there is as yet no cessation of nuclear division at 

 any surface region, cleavage may arbitrarily be said to terminate with 

 the arrival of the cleavage nuclei at the egg's surface. The embryonic 

 differentiations which immediately follow this stage and which lead up 

 to the completion of the blastoderm collectively constitute what is 

 herein termed blastulation. 



BLASTULATION 



At the conclusion of cleavage the egg may be said to be in the 

 blastema stage. This term was first used by Weismann (1863) to 

 designate the nucleated periplasm in the eggs of dipterans before its 

 division into cell territories. Although he was mistaken in assuming 

 that the nuclei in this layer have a spontaneous origin, later workers, 

 such as Patten ( 1884) and Wheeler (1889), have used the term with- 

 out objection. When the general definition of blastema, viz, the 

 primitive basis of a structure yet undifferentiated and from which 

 such a structure grows, is considered, it must be admitted that the 

 nucleated periplasm of insect eggs comes within the meaning of the 

 term when its function as the primitive basis for the blastoderm is 

 taken into account. For this reason, the precedent established by 

 Patten and Wheeler is herein followed. 



Four substages of the blastema are to be recognized in the eggs of 

 fleas, each determined by the number of nuclei which are present in 

 the periplasm. The first of these has already been described as being 

 synonymous with the 128 nucleus stage. The second, third, and fourth 

 blastema substages correspond to the 256, the 512, and the 1024 



