358 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



rudiment already mentioned, and the anterior amnioserosal fold there is 

 a simple emigration of cells from the blastoderm without the formation 

 of a perceptible groove. No proliferation of cells takes place but merely a 

 sinking in of the cells located at the mid-line. In the region adjacent to 

 the anterior rudiment the transition from the cells of the inner layer to 

 those of the rudiment is difficult to make out. Posteriorly the migration 

 of cells appears to become more regular. At the beginning of the second 

 third of the germ band there is a gradual transition from this method of 

 inner-layer formation to that in which the migration of cells is combined 

 with the formation of a shallow groove whose sides are destined to form 

 the inner layer. The sides of the groove approximate but never form a 

 tube in this region. At a point somewhat ventral to the posterior pole 

 of the egg, the groove deepens abruptly, and its sides widen out below 

 the surface layer, after which the lips of the groove approach each other 

 to fuse and form a tube with a distinct lumen. In the relatively short 

 region at the posterior end of the embryo, between the posterior mesen- 

 teron rudiment and the tubular section, a simple groove is again produced, 

 and finally in the terminal section the inner-layer formation is hke that 

 of the anterior third of the embryo. As has already been stated, the 

 mesenteron rudiments of the flea originate from near the anterior and 

 posterior ends of the germ band, before the formation of the inner layer 

 is evident. 



True segmentation of the germ band in the flea takes place the third 

 day of development when the germ layers are completely differentiated 

 and revolution has taken place. The procephalic lobes become divided 

 into three segmental regions, of which the most anterior develops into a 

 bilobed prominence, the anlage of the labrum; the second, into the 

 antennal segment; and the third, into the intercalary segment. The 

 protocormic region gives rise to 17 segments plus an additional incom- 

 plete one which is to be regarded as the telson. The three procephahc 

 and the three gnathal segments soon fuse to form the syncephalon of the 

 developing larva. Of the remaining segments the first three form the 

 future thorax. The small eleventh abdominal segment is soon carried 

 inward by the invagination of the proctodaeum, telescoping into the 

 tenth. In the later stages of development the embryo greatly elongates 

 and retains its curled position until the time of hatching after the sixth 

 day of development. 



The fore-gut and the hind-gut arise as ectodermal invaginations, the 

 former ultimately reaching about one-fourth the length of the egg. The 

 point at which the embryonic band sinks inward into the yolk is almost 

 immediately over the position of the posterior mesenteron rudiment and 

 therefore somewhat anterior to the posterior extremity of the germ band. 

 This is also the point at which the proctodaeum pushed inward so that 



