494 THE NERVOUS SYSTEM. 
stage of development there is a close resemblance between the 
nervous system of the Muscide and that of the Arachnidz, in 
which the hemispheres are more distinctly separated from the 
infra-cesophageal centres than the latter are from the thoraco- 
abdominal nervous mass. 
The Stomo-gastric Nerves and Ganglia. —I have been quite 
unable to follow the changes in the stomo-gastric nervous 
system, but have found five small ganglia closely related to the 
hemispheres in the resting larva (Pl. XXXIV., Fig. 2, g). These 
are probably the median and lateral ganglia of the stomo-gastric 
system, and are possibly, I think, developed from the hemi- 
spheres at a very early stage of their evolution. Viallanes, 
however, considers that they are developed, in Mantis, indepen- 
dently of the central nervous system, from cells which lie in 
close relation to the stomodaum, and I have no observations 
which throw any light upon the subject. 
Morphological Conclusions—The morphological conclusions at 
which I have arrived by a study of the brain of the Blow-fly in 
its various stages are: 
1. That there is a distinctly vesicular stage in the develop- 
ment of the preoral centres. 
2. That the floor of the vesicle forms the central ganglia, which 
I have distinguished as the pro-, meso- and meta-cerebron; but 
I have been unable to distinguish the exact manner in which 
each of these brain segments is derived from the three masses 
of white substance seen in early stages of development. 
3. That the ganglia of the special senses of smell and sight 
are developed from the roof of the vesicles, which also appa- 
rently give rise to the small cells of the corpora fungiformia. 
4. That the retina is developed from an involution of the 
roof of the vesicle of the cerebron, in relation with the great 
optic disc and the optic stalk of Weismann. 
5. That the nerves to the ocelli, the pyramidal ganglion, and 
all their nervous elements, originate from the roof of the 
vesicle. 
6. That the procerebral lobes probably arise from the 
anterior part of the roof of the vesicle. 
