NO. 3 EMBRYOLOGY OF FLEAS KESSEL 47 



elongated nature of the hind-intestine becomes evident rather early 

 in development. As a consequence of this elongation, the procto- 

 daeum, unlike the stomodaeum, soon becomes coiled and at some 

 stages is curved somewhat laterally to the midline. This is evidenced 

 by the fact that certain sagittal sections, as well as transverse ones, 

 cut the proctodaeum at more than one place (pi. u, figs. 80, 84). 

 Strindberg (1917) pointed out this elongated nature of the procto- 

 daeum in his brief paper on Archaeopsylla. He also notes that the 

 hind-intestine is considerably thickened near the anal opening, and 

 that it is one of the most conspicuous of the embryonic organs. 



Associated with the hind-intestine are the malpighian tubules. There 

 are four of these formed in the flea embryo. As in other insects, they 

 arise as diverticula of the proctodaeum proximal to its blind end. 

 Their first evidences make their appearance rather early, in fact 

 while the proctodaeum still lies perpendicular to the germ band. By 

 the time the proctodaeum becomes directed anteriorly they may be 

 easily seen (pi. 9, fig. 71). They grow very rapidly and soon show 

 as thick-walled tubes in cross-section (pi. 3, fig. 32; pi. 11, fig. 84), 

 lying laterad to the developing hind-intestine. Their blind ends remain 

 free in the haemocoel. 



The mesenteron derives its inner lining from the two entodermal 

 rudiments whose differentiations have been described in a previous 

 section. As in the majority of insects whose embryology has been 

 studied, this epithelium is entirely bipolar in origin, in contrast to the 

 condition described by Strindberg (1913) for Isoptera, in which he 

 says the mesenteron rudiment is single and proliferated from the entire 

 length of the germ band. This same author (1917), however, derives 

 the entoderm of fleas solely from an anterior and a posterior mesen- 

 teron rudiment. His error in deriving these rudiments from the 

 lower layer has already been pointed out. 



With the shrinkage of the yolk and the invagination of the stomo- 

 daeum and proctodaeum, the two entodermal rudiments are carried 

 into the interior of the embryo. At first they lie as small clumps of 

 cells against the blind ends of the ectodermal invaginations which 

 have pushed them inward. Because of this apposed relationship to the 

 ectoderm, many workers have come to the mistaken conclusion that 

 the inner lining of the midintestine is ectodermal in its derivation. 



The mesenteron rudiments, as already stated, lie slightly ventrad 

 to the blind ends of the stomodaeum and proctodaeum. From each 

 rudiment there is proliferated a pair of laterally placed tonguelike 

 processes, those of the anterior rudiment directed posteriorly (pi. J 1 , 

 figs. 81, 83), and those of the posterior one directed anteriorly. These 

 4 



