lOO WHEELER. [Vol. VIII. 



supraoesophageal mass, the protocei'eb^nmi^ or first brain seg- 

 ment, which constitutes the greater portion of the brain. 

 Each of its halves may be separated into three lobes ; the 

 first, or outermost lobe {pc^ \P-g-^ forms the optic ganglion of 

 the larva and imago, while the second and third lobes {pc-, 

 pc 3) ultimately form the bulk of the brain proper. The third 

 lobe is united with the contralateral lobe by the broad supra- 

 oesophageal commissure. Such is the structure of the 

 Orthopteran brain reduced to its simplest terms. It may now 

 be considered a little more in detail. 



Like the nerve-cord, of which it is simply a modified portion, 

 the brain arises from neuroblastic cells. These first make 

 their appearance in clusters (the spots seen on the procephalic 

 lobes in Fig. 2). Later they form a single layer of proliferat- 

 ing centres continuous with and in every way comparable to the 

 neuroblasts of the ventral nerve-cord. Like the latter they are 

 covered externally by a layer of dermatoblastic cells. 



That the deuto- and tritocerebral ganglia are strictly 

 homodynamous with the ganglia of the nerve-cord is clearly 

 shown in Xiphidiitm. In the first place these brain segments 

 are directly continuous with the segments of the cord; second, 

 they have at first the same size and shape as the latter, and 

 third, they present on the average four neuroblasts in cross- 

 section on either side. The suppression of the median cord in 

 the deutocerebrum (if it be not drawn forward into the proto- 

 cerebrum), is perhaps sufficiently explained by the presence of 

 the stomodaeal invagination. A partial suppression of the 

 median cord in the tritocerebrum may be due to the same 

 cause. The infraoesophageal commissure is perhaps the 

 morphological equivalent of both the commissures of a ventral 

 ganglion. 



The early clustered condition of the neuroblasts is seen in 

 Figs. 32-34 at nb. At the edges of these cross-sections a 

 rounded mass of pale cells {pc'^ \p-g^ is distinctly marked off 

 from a more deeply stainable layer which encloses it on nearly 

 all sides. This mass, the future optic ganglion (first proto- 

 cerebral lobe), is delaminated from the ectoderm at a very 

 early stage. The cells of the mass agree with the neuroblasts 



