276 ANIMAL BIOLOGY. [Part II. 



In the mid-line of the ventral plate a ventral groove is formed, 

 which, as it sinks below the general level of the epiblast, gradu- 

 ally detaches itself as a tube. The tube becomes a solid cellular 

 mass, which splits into two longitudinal bands, the mesoblastic 

 bands. 



The hypollast originates, in insects, according to some ob- 

 servers, by differentiation of the internal mass of yolk-cells; 

 according to others, by differentiation of certain cells of the 

 ventral groove. 



The mesoblastic bands become divided into somites, each of 

 which becomes hollow, the hollowed space separating an inner, 

 dorsal, one-layered wall from an outer, ventral, several-layered 

 wall adjoining the epiblast. By the coalescence of the cavities 

 of contiguous somites a common cavity the cwlom is formed. 

 The coalesced inner walls of the somites give rise to a splanchnic 

 layer; the coalesced outer walls to a somatic layer. Certain 

 yolk-cells seem to pass into the ccelom, and it has been sug- 

 gested that these give rise to the corpus adiposum, or fat 

 body. 



The nervous system is epiblastic in origin. The cerebral 

 ganglia are developed separately as a pair of pre-oral thickenings 

 of the epiblast. The post-oral chain is developed from longi- 

 tudinal thickenings of the epiblast, which sink into the body 

 and become separated from the superficial epiblastic layer. 



All the anterior part of the alimentary canal (fore-gut), lined 

 with a chitinous infolding from the edges of the mouth, is 

 formed by a stomodceal imagination. All the posterior part of 

 the alimentary canal (hind-gut), as far as, and including, the 

 Malpighian tubules, with infolded cuticle from the vent, arises 

 by a proctodceal invagination. The chylific stomach and its caecal 

 tubes (mid-gut), develops from the mesenteron. The muscular 

 layer of the alimentary canal is contributed by the splanchnic 

 layer of the mesoblast. 



Perhaps the most striking peculiarity in the development of 

 insects is the formation of an embryonic membrane similar to 

 the amnion. A fold of membrane grows out from the blasto- 

 dermic layer over the germinal plate, and, coalescing in the 



