66 THE LARVA OF THE BLOW- FLY. 



ganglionated plexuses in the walls of the viscera, similar to 

 those of Auerbach and Meissner in the vertebrata. 



b. Structure of the Nerve Centres. 



The somatic nerve-centres in all insects consist of a central 

 stroma of fibrillated substance, which I regard as analogous to 

 the network of Gerlach, surrounded by a vesicular layer, or 

 'gray substance.' This is enclosed in a thick capsule of 

 mesoblastic tissue rich in tracheae — the peritoneal capsule — 

 continuous with the sheaths of the nerves. 



The Central Stroma consists of a fine network of axis 

 cylinders, permeated in many cases by distinct bundles of 

 larger fibres — commissural fibres, and nerve roots. 



Many of the nerves arise in part from this stroma, and in 

 part from the peripheral nerve-cells. Some portions of the 

 white substance are blackened very easily by osmic acid, and 

 others are scarcely tinged by it. Those parts which become 

 black are infiltrated by an interfibrillar substance of a fatty 

 character. Dietl* named this variety of the stroma ' Mark- 

 substanz,' medullated stroma, or medullated substance. 



The medullated substance has been a great difficulty in the 

 investigation of the nervous system in insects. It often appears 

 in sections as a homogeneous or laminated material, or in the 

 form of solid balls, which have been mistaken for giant cells. 

 Dietl correctly described this substance. He says : ' Like ordi- 

 nary stroma, it consists of nerve fibrils, which var}^ greatly in 

 size — from large distinct nerve-fibres, such as are found in 

 nerves, forming commissural bundles, to the finest reticulum of 

 minute axis cylinders, forming a dense stroma ;' but he also 

 says ' it sometimes exists as a hoiriogeneous mass, or in the 

 form of laminae.' 



Such homogeneous masses are, however, readily resolved 

 into a stroma by treatment with a mixture of ether and chloro- 

 form, which dissolves a fatt}' interstitial substance. By this 

 method they are seen to differ in no way from the parts in 



* Dietl, ' Die Organization des Arthropodengehirns.' Zeitsch. f. w. Zool., 

 Bd. xxvii., 1876. 



