THE DEVELOPMENT OF THE NERVOUS SYSTEM. 483 
and 9). The shortening of the ganglionic chain, so charac- 
teristic of the Muscide and their allies, occurs during the 
second stage of development in the egg,—during the trans- 
formation of the nymphoid into the vermiform embryo. 
Development of the Preoral Centres.—It has been sufficiently 
established that the preoral centres originate from the pro- 
cephalic lobes, and that the latter are the anterior extremities 
of the two lateral halves of the primitive band ; but previously 
to the appearance of Viallanes’ memoir on the development of 
Mantis [190] no detailed description of the manner in whith 
the hemispheres of the brain are developed had been published. 
A perusal of Viallanes’ paper has convinced me that the 
development of the brain in the Blow-fly is very similar to 
that of Mantis, except that the process, which occurs in Mantis 
in the egg, is prolonged in the Blow-fly throughout the larva 
and earlier pupal stages ; indeed, Viallanes’ sixth stage of the 
development of Mantis in the egg closely corresponds with the 
completion of the formation of the nymph in the pupa of the 
Blow-fly during the third or fourth day; while his tenth and 
last stage of the Mantis embryo presents a nervous system 
closely resembling that of the Blow-fly on the seventh or eighth 
day of the pupa stage. 
Our knowledge of the first stages of the development of the 
brain are still exceedingly incomplete; but the structure of the 
hemispheres in the larva of the Blow-fly affords valuable indica- 
tions of the probable changes which occur in earlier stages; I 
shall therefore commence by a description of the parts in the 
adult larva, before the changes which occur in the resting stage 
of the larva take place. 
The Hemisphere of the Adult Larva exhibits a complex central 
stroma divided into several well-defined lobes, surrounded by a 
thick layer of cellular cortex, which presents various characters 
in different parts 6f the hemisphere. The cellular cortex is 
invested by a well-marked layer of columnar cells parti- 
ally covering the hemisphere and dipping into its substance, 
Separating the optic ganglion and the posterior lobes, which 
become the corpora fungiformia, from the remaining structures 
