DEVELOPMENT. 



191 



cellular layer, and thus forming the thin membrane which 

 invests the central nervous mass (fig. 112, JV"). The above- 

 mentioned solid mesoblastic band, which exists for a very 

 short time only, may perhaps be homologised with the 

 chorda dorsalis of Vertebrates, and the chorda of the higher 



Fig. 112. Transverse section of Nerve-cord of Embryo of B. germanica (Oc. 4, Obj. 

 D.D. Zeiss). (7, cellular layer; F, fibrillar substance ( jjunkt-substance of 

 Leydig) ; Ch, cellular band ; N l N n inner and outer neurilemma. 



AVorms, since in these types also the chorda forms a solid 

 cellular band of meso-hypoblastic origin, lying between the 

 nervous system and the hypoblast. The peripheral nerves arise 

 as direct prolongations of the fibrillar substance of the nerve- 

 cord. 



st 



pr 



Fig. 113. Alimentary Canal of Embryo of B. germanica. Copied from llathke, 

 loc. cit., but differently lettered. st, stomodseum, already divided into 

 oasophagus, crop, and gizzard ; m, mesenteron ; pr, proctodseum, with Malpighian. 

 tubules (removed on the right side). X 12. 



