4-;4 THE NERVOUS SYSTEM. 



stage of development there is a close resemblance between the 

 nervous system of the Muscidse and that of the Arachnids, in 

 which the hemispheres are more distinctly separated from the 

 infra-ossophageal 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 (PI. 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 stomodseum, 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 ,tage 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 fungi for mia. 



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 

 aid 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. 



