386 IXSECTA. 



The Nervous System. 



The central parts of the nervous system pass directly over from 

 the larva to the imago, although they undergo considerable modifica- 

 tions of form and position. At the same time certain histological 

 changes, known as histolysis, are said by Weismanx to take place in 

 them, e.g., a disintegration and reconstruction of the tissues within 

 the organs without disturbing their continuity. Recently, however, 

 the term hystolysis has been applied to the disintegration of the 

 tissues of the pupa generally. 



"VVe have as yet little light on the question of the transformation 

 of the peripheral nervous system. Although it must be considered 

 probable that the destruction of the larval muscles is accompanied 

 to some extent by a degeneration of the motor nerves, this is not 

 the case with the nerves that run to the extremities. These can 

 be recognised in the larva in the form of nerve-strands connect- 

 ing the imaginal discs with the central nervous system. These 

 strands, according to Van Rees, pass over from the larva to the 

 pupa and imago, so that, when the limb-rudiments develop further, 

 only the distal parts of the nerves belonging to them appear as new 

 formations. 



The Fat-body. 



The fat-body of the larva also is destroyed through the action 

 of the phagocytes in the way described in connection with other 

 tissues. Its reconstruction appears to proceed from the mesoderm 

 of the imaginal discs. It is possible also that the accumulations 

 of embryonic cells, assumed by Schaffer to be formative centres, 

 have to do with the regeneration of the fat-body. In any case it is 

 to be derived from mesodermal tissue. Even though \Vielowiejsky 

 observed the origin of the fat-body in Corethra from a cell-layer 

 lying beneath the hypodermis of the larva, such an observation does 

 not necessarily support the view of Schaffer, who thought he had 

 convinced himself that, in Musca, the fat-body of the larva is 

 derived partly from the hypodermis and partly from the tracheal 

 matrix, and thus from ectodermal tissue. 



The ultimate fate of the Phagocytes. 



We have seen that the development of the imaginal organs, in 

 cases where these are not taken over direct into the pupa, always 

 proceeded from the imaginal discs. The phagocytes, the number 

 of which increases greatly in the pupa, do not (as was formerly 



