THE NERVOUS SYSTEM. 



327 



The separation of the brain-rudiment from the ectoderm, like that 

 of the lateral cords, takes place by a process of delamination. An 

 exception to this rule is found in the composition of the optic 

 ganglion, which is formed from the invagination described above. 

 The fibrous tissue of the brain develops here as in the lateral cords, 

 first on the inner surface of the brain-rudiment, and only later sinks 

 into the interior of the brain and becomes surrounded with a layer 

 of ganglionic cells. 



The rudiments of the two halves of the brain are originally 

 distinct from one another. Later, when the dorsal part of the head 

 has formed, the two halves of the brain shift on to the dorsal side, 

 approaching one another until, finally, with the assistance of a 

 median invagination (like the transverse commissures of the ganglia 

 of the ventral cord), a commissural connection is established between 

 them (Grassi, No. 32 ; Heider, No. 38 ; Graber, Nos. 28 and 30). 



The most important recent details of the development of the brain in the 

 Insecta have been given by Patten (No. 67) for Acillus, and by Viallanes 

 (No. 84) for Mantis. According to Patten, the whole head-rudiment shows 

 signs of being composed of three segments (Fig. 160), this segmentation affecting 

 not only the primary parts of the brain-rudiment, but also the rudiments of the 

 optic ganglion and of the optic plates. 

 On the three consecutive segments 

 into which the optic plate (op) is thus 

 divided, the rudiments of the six 

 ocelli of the larva are distributed in 

 Acilius, two ocelli occurring on each 

 segment (Fig. 160 A, 1-6). In the 

 shifting of the separate parts of 

 the rudiment of the head, which 

 takes place at a later stage in con- 

 nection with the development of 

 the cephalic terga, as above men- 

 tioned (p. 302), changes occur in the 

 position of the ocelli with regard to 

 each other, but these we cannot here 

 enter upon. The invagination above 

 described also, which participates in 

 the formation of the optic ganglion, 

 is broken up, according to Patten, 

 into three sections corresponding to 

 the segmentation of the brain (Fig. 

 160 B, i 1 , ir) ; in Acilius, only the 

 two anterior sections can be recognised 

 as distinct invaginations, while the 

 third is replaced by a solid ingrowth. 



Patten's statements have been almost entirely confirmed by Wheeler 

 (No. 95) in the case of Doryp>hora. Cauriere's (No. 13) observations also 



- qo 



Fig. 161. — Anterior (ventral) aspect of the 

 developed brain of Oedipodtt (after Vial- 

 lanes). c, circum- oesophageal commis- 

 sure ; c', transverse commissure behind the 

 oesophagus ; do, deutocerebrum ; go, optic 

 ganglion; If, labro-frontal nerve; na', an- 

 tennal nerve ; na", accessory antennal 

 nerve ; no, nerves of the three ocelli ; pc, 

 protocerebrum ; r, root of the paired 

 stomato-gastric ganglion ; tc, tritocerebrum. 



