THE DEVELOPMENT OF THE NERVOUS SYSTEM. 491 



is exceedingly complex. Not only is the central stroma divided 

 into distinct lobes even in the recently-hatched larva, but the 

 two great commissures, one below and the other above the 

 resophagus, are already developed. In the adult larva the 

 optic ganglion is not only recognisable, but its central stroma 

 already exhibits the radial and concentric structures so charac- 

 teristic of the same part in the imago, and the cells of the 

 pyramidal ganglion and of the olfactory ganglia are readily 

 distinguished. In Plate III. two sections of the brain of the 

 larva are represented at a somewhat more advanced stage of 

 development than those which are now published. In Fig. I. 

 the section is entirely behind the optic ganglion, and exhibits 

 the trabeculae and corpora fungiformia ; and in Fig. II. it is in 

 front of the optic ganglion, and shows the two ossophageal 

 commissures. In several of the sections in the series from 

 which these two figures are taken, the optic ganglia are dis- 

 tinctly recognisable ; but owing to these sections having been 

 broken, they were not figured, and the appearances which they 

 presented puzzled me much at the time, as I was then un- 

 acquainted with the manner in which the optic ganglia are 

 embedded in the substance of the hemispheres. 



Changes of the Neuroblast in the Pronymph. The first change 

 which attracts our attention in the neuroblast in the early 

 stages of pupation is the rapid increase in the size of the hemi- 

 spheres. This is accompanied by a similar increase in the 

 rudiments of the infra-cesophageal ganglia, which become 

 segmented from the anterior part of the ventral cone ; and by 

 a rapid increase in the size of the cesophageal connectives. 



The hemispheres at the same time become ovoid, with their 

 long axes transverse to the body. This elongation is due to 

 the rapid increase in the size of the opti^ ganglia, which pro- 

 ject more and more from the surface of the hemispheres. 



Weismann [2], describing the changes which occur on the 

 third day of the pupa, states that : ' Each hemisphere, which 

 during the first two days underwent an extraordinary enlarge- 

 ment, exhibits a moderately-deep circular groove on its surface, 

 separating a median from a lateral ganglion. These both 



