368 THE TRACHEAL SYSTEM Ul' Till: IMAGO. 



they do not enter the muscle-fibres and do not generally 

 penetrate the epithelial walls of the body or viscera. 



There are, however, some exceptions to this : trachea pene- 

 trate the nerve-centres, and dioptron ; ramify between the large 

 epithelial cells of the rectal papillae, and also penetrate the 

 thicker portions of the proventriculus. 



I know no other parts in which the tracheae enter the 

 epithelial layer. Whenever they do there are distinct pro- 

 longations of the reticular tissue of the ccelom between the 

 epithelial elements. 



3. THE DEVELOPMENT OF THE TRACHEAL SYSTEM IN 



THE BLOW-FLY. 



The manner in which the first tracheal trunks are developed 

 in the embryo has been already described, but, owing to the 

 difficulties which beset the investigation, it is satisfactory to 

 find that the subsequent changes, which occur in the larva and 

 pupa, bear out the views of Weismann on the development, 

 and of Gegenbaur on the morphology of the tracheal system. 



Development in the Larva. As Weismann observed [2, p. 115] 

 the tracheal system is profoundly modified during the growth 

 of the larva and the development of the nymph. In the larva 

 the changes in the tracheal system are : (i) a continual increase 

 in the number of arborescent tracheae, together with a corre- 

 sponding increased complexity of the tracheal network, which 

 invests all the internal organs and supplies the hypodermic 

 layer of the integument; (2) The development of the anterior 

 spiracles or stigmatic cornua ; and (j) The re-developrr.ent of 

 the posterior spiracles. 



New Ramifications of the Tracheae first appear as buds or solid 

 outgrowths of the peritoneal coat of the old ones ; these buds 

 become hollow and are at first filled with fluid; a new intima 

 is then developed, but no air finds its way into them until the 

 next ecdysis, when the intima of the new branches is found to 

 be continuous with the new intima of the older trunks. The 

 capillaries are developed in the interior of the stellate cells of 



