479 



and downwards and enclose between them the oesophagus 



(fig- 231). 



The brain at this early stage is not in a very advanced 

 condition, and it may be divided into two parts, an inner 

 functional region and an outer region, in which active func- 

 tioning does not evidently occur (fig. 230). The functional 

 (truly larval) portion of the brain consists of a mass of nerve 

 cells, occupying a gre^at part of the interior of the brain. The 

 individual nerve cells a'p'pear to be quite small, seldom more 

 than 5/x in diameter ; this is due to the fact that most of their 

 cytoplasm is to be found in the long nerve fibres, whose 

 volume it is not possible to estimate accurately. They have a 

 faintly granular nucleus; the usual karyosome is very small 

 or often quite absent. 



The fibres from these nerve cells all converge to form a 

 pair of great nerve tracts, one on either side, within the 

 brain, and these great nerve tracts are joined by a very 

 narrow tract from the inner portion of the antero-ventral 

 brain region — from the antennal ganglion. Other nerve 

 fibres from this antennal ganglion unite to form a very 

 minute nerve which terminates on the pair of minute sense 

 papillae (true antennae) of the first segment. In this region, 

 and also within the great central mass of nerve tissues, 

 synapses must occur in great numbers, but I can say nothing 

 definite about them here. Some of the nerve fibres in the 

 brain cross to the opposite side, others form strands which 

 travel in various directions. From the brain numerous other 

 fibres collect to form the two' circumoesophageal nerve tracts, 

 which connect the brain with the ventral ganglia. 



Forming a distinct layer on the outside of the functional 

 nerve cells are the neuroblasts, evidently non-functioning cells 

 at this period of development (fig. 230). They are 8/i to 9/a 

 in diameter, and have the ''vesicular" type of nucleus with 

 its large karyosome, so commonly found among undijfferenti- 

 ated cells. Though they appear to be larger than the func- 

 tional nerve cells, this is in reality not so, most of the cyto- 

 plasm of the latter being found in the long nerve fibres; in 

 4his respect, then, they form no exception to the rule that 

 the functional larval cells are always much larger than the 

 non-functional imaginal cells, which will replace them during 

 metamorphosis. The neuroblasts form especially well-devel- 

 oped areas in certain parts of the brain : there is a pair of 

 very well-defined layers, in places swollen into large masses, 

 on the outer lateral regions, constituting the anlagen of part 

 of the two optic ganglia (fig. 230). They extend round partly 

 behind the brain as large bowl-shaped structures and give 

 ([off forwards each a small mass of cells which projects into the 



