76 THE LARVA OF THE BLOW- FLY. 



consists of one or more layers of cells, those on the surface, 

 which is enclosed by the provisional capsule, are distinctly 

 columnar, those beneath, nearer the mesoblastic surface, are 

 small round or fusiform cells. The mesoblastic surface is 

 usually concave, and its cavity is occupied by a layer of stellate 

 mesoblast. 



The great discs of the head and the inferior pro- and meso- 

 thoracic discs are so intimately connected with the neuroblast 

 by neural stalks, and are apparently so far removed from the 

 hypodermis, that it is not difficult to understand how Weis- 

 mann and Ganin arrived at the conclusion that they originate 

 from the sheaths of the nerves. 



The rudiments of these discs certainly appear in the embryo, 

 close to the roots of the nerves to which they become 

 adherent, whilst the nerve-centres are in contact with the 

 ventral integument : as the growth of the larva is rapid 

 whilst that of the neuroblast and discs is slow, the latter 

 soon become separated from the hypodermis ; and the 

 pouches which contain the rudiments of the discs are drawn 

 from their points of origin, leaving the neck of the disc-sac as 

 a very narrow tube attached to the growing peripheral portion 

 of the corresponding nerve. The neck of the sac is not readily 

 seen, hence Weismann regarded the bands which he saw in the 

 adult larva connecting the discs with the integument as nerves 

 or of secondary import. 



The portion of the nerve between the disc and the neuro- 

 blast becomes greatly enlarged towards the end of larval life, 

 owing to the rapid multiplication of the mesoblastic elements 

 of its sheath, which are continuous with the mesoblast of the 

 disc. 



The epiblast of the disc, as development progresses, usually 

 becomes much folded or invaginated into itself; its cells are 

 then found united by an intercellular substance resembling 

 chitin, and one or more thin layers of cuticle are often shed 

 from its surface. The disc-sac or provisional layer of Ganin 

 consists of thin tessellated cells, or may exhibit transitional 

 characters both at the edge of the disc and in the vicinity of 



