THE DEVELOPMENT OF THE NERVOUS SYSTEM. 483 



and g). The shortening of the ganglionic chain, so charac- 

 teristic of the Muscidas 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 which 

 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 of 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 



