192 THE COCKROACH: 



Alimentary Canal. The epithelium of the mesenteron is 

 formed out of the hypoblast, whose cells assume a cubical form 

 and gradually absorb the yolk. The epithelium of the stomo- 

 daourn and proctodacum is derived, however, from two epiblastic 

 involutions at the fore and hind ends of the embryo. The 

 muscular coat of the alimentary canal is contributed by the 

 splanchnic layer of the mesoblast. The mesenteron in an 

 early stage of development appears as an oval sac of greenish 

 colour (fig. 113), faintly seen through the body- wall. The csecal 

 tubes are extensions of the mesenteron, the Malpighian tubules 

 of the proctodcToum. The epiblastic invaginations may be 

 recognised in all stages of growth by their chitinous lining 

 and layer of chitinogenous cells, continuous with the similar 

 layers in the external integument, 



Tracheal System. Tubular infoldings of the epiblast, forming 

 at regular intervals along the sides of the embryo and project- 

 ing into the somatic mesoblast, give rise to the paired tracheal 

 tubes, which are at first simple and distinct from one another.* 



Heart. The wall of the heart in Insects is of mesoblastic 

 origin, and develops from paired rudiments derived from that 

 peripheral part of each mesoblastic band which unites the 

 somatic to the splanchnic layer. In this layer two lateral 

 semi-cylindrical rudiments appear, which, as the mesoblastic 

 bands meet on the dorsal surface of the embryo, are brought 

 into contact and unite to form the heart (fig. 111). The heart is 

 therefore hollow from the first, its cavity not being constricted 

 off from the permanent perivisceral space enclosed by the 

 mesoblast, but being a vestige of the primitive embryonic 

 blastoco3l, which is bounded by the epiblast, as well as by the 

 two other embryonic layers. Such a mode of the development 

 of the heart was observed by Biitschli in the Bee, and by 

 Korotneff in the Mole Cricket. I am convinced, from my own 

 observations, that the heart of the Cockroach originates in this 

 way, though it is to be observed that, in consequence of 



* In Gryllotalpa (Dolirn), as in Spiders, some Myriopods and Peripatus (Moseley, 

 Phil. Trans., 1874), each stigma, with its branches, constitutes throughout life a 

 separate system. The salivary glands arise in the same way, not, like the salivary 

 glands of Vertebrates, as extensions of the alimentary canal, but as independent 

 pits opening behind the mouth. Both the tracheal and the salivary passages are 

 believed to be special modifications of cutaneous glands (Moseley). 



