THE SO MA TIC NER VO US S YS TEM. 69 



larva i'5 centimetres long, it only measures 74 to 78 mm. All the nerves 

 originate from the ventral cone, and there are twelve pairs, two from the 

 front and ten from the sides.' 



In this statement Weismann evidently made a slight error, as he subse- 

 quently describes the origin of the optic nerves from the hemispheres ; more- 

 over, a pair of antennal nerves arise from them : both these are, however, 

 remarkable, as they are the nerve-stalks of the antennal and optic discs 

 and although undoubtedly nerves, inasmuch as they contain nerve fibres, 

 they consist, like all the nerve-stalks of the imaginal discs, principally of 

 embryonic tissue. 



Weismann continues : 'The hemispheres and ventral cone consist of a 

 thin, tough external capsule and its cellular contents ; the cells are like the 

 nerve cells of most insects : small and spherical and without visible processes ; 

 they lie close together, and are arranged in no definite order.' 



In these statements Weismann was only correct in part, the cells he saw 

 were the embryonal elements, and he had not the means at his disposal the 

 preparation of thin sections which would have enabled him to arrive at 

 more correct conclusions. 



He further states that ' the nerve centre exhibits a clear cortex and a 

 dark medulla, and is one of the few examples of a tissue permeated by 

 tracheal vessels. In each hemisphere a tracheal trunk passes, without 

 dividing, almost into the centre of the organ, and then gives off a number of 

 fine radial branches. In the ventral cone the greater part of the tracheal net 

 lies on the surface, only in the middle line of the dorsal region a few air- 

 vessels pass into its substance ; these perforate the nerve-mass in a vertical 

 direction and give off branches in a stellate manner to a limited region in its 

 substance.' 



I shall not give a very detailed account of the neuroblast 

 in this section, as it will be more convenient to do so after 

 describing the central nervous system of the imago, in the 

 section devoted to its development, but shall content myself by 

 describing its most salient features. 



The Capsule consists of a thick layer of mesoblast formed of 

 a reticulum of stellate cells, and is richly supplied with tracheal 

 vessels. It not only covers the surface of the neuroblast, but 

 dips in between its constituent ganglia (PL III., cc), more 

 especially in the median line and around the base of the hemi- 

 spheres an arrangement which accounts for the penetration 

 of the tracheal vessels into the interior of the neuroblast. 



The Cortical Substance consists in great part of embryonic 

 cells, but stellate and fusiform cells are found in great numbers 

 in the immediate vicinity of the nerve - roots around the 

 oesophagus, and on the dorsal surface of the abdomino-thoracic 



