488 



THE NERVOUS SYSTEM. 



an earlier stage in which he says no central cavity exists ; and 

 this apparently led him to regard the central cavity as possibly 

 due to shrinkage during the hardening process. 



The existence of one or more ventricles in the Arthropod 

 brain has already been mentioned, so that it is possible that 

 a more general cavity exists at an early stage of development. 

 In some of Viallanes' figures of the earlier stages of the develop- 

 ment of the preoral nerve-centres of Mantis there are undoubted 

 cavities [190, Figs, u, 13 bis, 14], but he regards these as the 

 result of a partial separation of the neural and dermal rudiments. 

 A comparison of the later stages renders it extremely improb- 

 able, however, that the thick layer which he regards as dermal 

 in the young embryo is reduced entirely to the thin layer which 



!'!';. 65. A diagram representing the probable relations of the prestomal ganglia 

 and proccphalic lobes at an early stage, c t r.,, cavities formed by the delamina- 

 tion of the epiblast ; //, hemisphere; in, mesoblast ; //,, neural crest of the 

 hemisphere ; ;/.,, neural crest of the ventral ganglion ; /, pericardial space ; 

 //', primitive band ; //, procephalic lobe ; s, stomoihvum. 



he represents in the later stages, so that I regard it as by no 

 means improbable that a large ventricular cavity exists at an 

 earlier stage of development, and that the roof or dermal layer of 

 this cavity becomes differentiated into the mantle which covers 

 so large a part of the hemisphere in its earlier stages, whilst 

 the central ganglia and the white substance originate in that 

 portion of the primitive brain which forms the innermost part of 

 the wall of the vesicle, and which is from the first continuous 

 with the M'gmental ganglia of the ventral cord. 



