68 THE LARVA OF THE BLOW-FLY. 



system of the larva^ and in part of embrj'onic structures 

 destined to form the nerve centres in the nymph. 



AUhough these parts are intermixed in a complex manner, 

 the cellular elements of each are distinctly recognisable, and 

 those which are active in the larva undergo degeneration, like 

 the rest of the larval tissues, whilst those which are embryonic 

 in type undergo evolution during the formation of the nymph. 



As the embryonic portion predominates over the active nervous 

 elements, I propose to speak of the whole as the ' neuroblast,' 

 as a more appropriate term than ' central nervous system of 

 the larva,' although the latter forms a portion which is easily 

 recognised, but cannot be very definitely limited, except b}' a 

 most detailed and elaborate description. 



The Neuroblast is formed by the fusion of the primitive 

 ganglia of the embr3'o ; these undergo partial differentiation into 

 nerve-cells and stroma, and remain more or less separated by 

 mesoblastic tissue, so that tracheal vessels are found in the 

 substance of the organ. A large part of the cephalo-thoracic 

 ganglia remains embryonic, whilst a smaller portion of the 

 abdominal, and that only in the more anterior ganglia, retains 

 embryonic characters. 



I am not aware that any close investigation of this remark- 

 able organ has been made by any previous writer, and with 

 the exception of a description of the manner in which the optic 

 lobes of the imago are evolved, by Viallanes, to which I shall 

 have occasion to refer subsequently, I have been unable to find 

 any more detailed description than that which Weismann pub- 

 lished in 1864. He says : 



' The central portion of the nervous system of the larva difters remarkably 

 from that of other insects, inasmuch as there is no ventral cord, but a conical 

 nerve mass, the virtual construction of which, from the ganglionated cord, is 

 not betrayed even by a vestige of lateral constrictions. 



'The infra-cesophageal ganglia are intimately fused with those of the 

 thorax and abdomen, whilst the pre-oral ganglia assume the form of " hemi- 

 spheres," are nearly spherical, and are united with the ventral cone by short 

 thick crura, leaving only a narrow space between them for the passage of the 

 oesophagus. The " hemispheres " lie on the dorsal aspect of the ventral cone, 

 so that seen in profile the whole resembles the butt-end of a pistol. 



'The central nerve-mass is about one-twentieth of the body length — in a 



