481 



dividing cells may now be clearly distinguished : there are — 

 (a) the large rather strongly stainiAg cells which form the 

 various ganglia and the "bourrelet intraganglionaire" 

 ("Zweite Bildungsherde" of Bauer), and (h) a great mass of 

 more rounded paler cells sometimes still seen in mitosis and 

 forming the greater part of the imaginal brain where the 

 great ganglia do not occur. A small group of these is to be 

 seen between the optic ganglion and the zone of degenerating 

 larval cells, into which they project. It is possible that these 

 constitute a mass of neuroglia cells ; the proliferation of others 

 which form a great ring around the bases of the two hemi- 

 spheres in the neighbourhood of the ooellar and antennal 

 ganglia, is resulting in a gradual constriction of the great 

 central mass of degenerating nerve fibres here (cf. fig. 232, x). 



In the larva eight hours after def aecation a small mass of 

 cells, often in active mitosis, appears outside the extremity 

 of the degenerate nerve strand. It seems to be formed as 

 an ingrowth from the optic ganglion and constitutes the 

 "bourrelet perilaminaire" of Viallanes, the ''(Erste) Bildungs- 

 herde" of Bauer (fig. 232 o.g.2). 



From the simple anlage of the optic ganglion three masses 

 of cells therefore arise : — 



(1) Those which form what I shall call the outer optic 

 ganglion, whose fibres communicate with the compound eye. 

 It is that part of the primitive anlage which remains when 

 the other two parts have been formed from it, and occupies 

 the position of the "optic ganglion," as I have referred to it 

 above (o.g. in figs. 80 and 236). 



(2) Those which form the middle optic ganglion, as I shall 

 call it, and are represented by the "Bourrelet perilaminaire" 

 of Viallanes. This mass first becomes visible in the larva 



-eome eight hours after defaecation (o.g.2 in figs. 80, 232, 

 236). 



(3) Tliose which will form the inner optic ganglion, and 

 correspond to Viallanes' "Bourreliet intraganglionaire" (o.g. 3 

 in figs. 80, 232, 236). 



During larval life there is a continued growth in the 

 Vsize of the brain, but it is in the last few hours that it begins 

 I to assume its adult appearance. This takes place in three 

 ways : (a) by the gradual change in shape of the optic ganglia, 

 (h) by the development of nerve fibres and synapses, (c) by 

 'ithe gradual incorporation of the first ventral (sub-oesopha- 

 geal) ganglion. 



Towards the end of the larval period the cells comprising 



lihe outer optic ganglion begin to migrate outwards in the 



Ifmeshwork of fibres formed by the perioptic membrane (see 



