NO. 3 EMBRYOLOGY OF FLEAS KESSEL 43 



The true segmentation of the germ band of fleas does not take place 

 until the third day of development when the germ layers are com- 

 pletely differentiated and revolution has taken place. This circum- 

 stance differs, therefore, from that which occurs in other insects such 

 as HydropJiihiS (Kowalevsky, 1871), Chalicodoma (Carriere, 1890), 

 and Apis ( Nelson, 1915), in which the definitive segments are marked 

 off even before the lower layer is formed. 



Preliminary to the segmentation of flea embryos, therefore, the 

 double-layered germ band is produced. A description of this process 

 as seen in sections has already been given. However, an understanding 

 of its superficial appearances is requisite to the description of the 

 external manifestations of segmentation. The superficial indications 

 of the germ band become evident during the second day of develop- 

 ment. The first of these consists of a flattening of the middle portion 

 of the ventral plate preliminary to the immigration of its cells to form 

 the entoderm and mesoderm. Next, the anterior region of the ventral 

 plate expands perceptibly. This is coincident with the lateral expan- 

 sion of the mesoderm. Evidencing this is the fact that the germ band 

 becomes distinctly more opaque than the lateral regions of the blasto- 

 derm which are destined to form the embryonic envelopes. The 

 lateral expansion of the anterior part of the germ band serves to 

 differentiate the embryonic rudiment into two general regions (pi. 7, 

 fig s - 55> 5/)- The expansions of the anterior shorter one are the 

 procephalic lobes (pi. 9, fig. 68). The longer narrower portion is the 

 protocormic region. When the procephalic lobes first make their' 

 appearance, they are situated somewhat ventrad to the anterior pole 

 of the egg. With the lengthening of the germ band, however, they 

 come to lie dorsad to this pole, in which position they are to be found 

 at the middle of the third day of development. At this time the caudal 

 portion of the embryo has completely withdrawn from the yolk and 

 the two ends of the germ band almost touch. 



In some insects there is a pronounced difference in time between the 

 appearance of the anterior and the posterior segments of the germ 

 band. Those of the cephalic region have been reported to appear 

 earlier in the majority of forms. Schnetter (1934) for Apis and 

 Butt (1936) for Brachyrhinus have found that segmentation is 

 initiated, not at the extreme anterior end, but in the future thoracic 

 region from which point it extends both anteriorly and posteriorly. 

 In flea embryos the difference in the time of appearance of the seg- 

 ments in the various portions of the germ band is negligible, but the 

 anterior-posterior sequence is followed in this discussion for the sake 

 of convenience. The procephalic lobes become divided into three 



