OF OECANTHUS AND TELEAS. ■ 251 



tion to the invaginations of the thoracic and abdominal segments which form the tracheae ; 

 but at present I am unable to say whether these ingrowths form any portion of the tra- 

 cheal trunks, as they do in Lepidoptera according to Hatschek, or become transformed 

 into chitinous rods forming part of the internal skeleton of the head, as TichomirofF has 

 describedfor Bombyx mori. They probably disappear altogether, as no trace of them 

 was to be found in sections of an embryo about the time of hatching. A final decision on 

 this point is reserved until further study can be made. In the embryo at the time of 

 hatching there are two main tracheal tubes which extend along the sides of the body from 

 the end of the abdomen where they are smallest, into the thoracic segments. The 

 stigmatic openings, as well as the tracheae which supply the organs of the body, are all 

 connected with these trunks. 



The respiratory function of the embryo is first indicated at the time of revolution by 

 the appearance of paired lateral outgrowths of the ectoderm from the pleural region of the first 

 abdominal segment. These gills or respiratory organs come to lie just behind, but dorsad 

 of the base of the third thoracic appendage. (PI. 19, figs. 1, 17 ; pi. 22, figs. 13 and 14 ; 

 pi. 25, fig. 29.) In outline they are broadly oval or kidney-shaped and are united to 

 the body by a short peduncle springing from the centre of that face of the disc which is in 

 contact with the body of the embryo. These folds are cellular structures and at different 

 periods are solid or hollow. The cells of the folds early lose their ectodermic characters and 

 become somewhat larger than those of the adjacent body wall. In the fresh condition 

 they appear enucleate and coarsely granular, but upon treatment with osmic ora cetic cid 

 a nucleus is distinctly visible. In surface view there is to be seen a clear central area 

 which indicates the position of the internal cavities of the gill. These cavities are contin- 

 uous with the body cavity and probably serve as channels through which the vascular fluid 

 circulates. They vary in shape and relative proportions. The relations of these append- 

 ages to the body is best seen in sections. (PI. 22, figs. 13, 14.) The out-growing flap is 

 here seen to project over an invagination immediately below it and in some instances to 

 become apposed so closely to the body wall as to convert the open pocket into a closed 

 canal. In its middle part, where the fold fuses with the body, its cells are separable into 

 two irregular layers which correspond to the two primitive plates of the fold, but they 

 fuse completely, or become widely separated, in the free portion of the pad. These append- 

 ages reach their greatest degree of development soon after the revolution of the embryo, 

 and then gradually atrophy, entirely disappearing before the complete closure of the body 

 walls. In sections of the gill organ before its atrophy (or absorption) one finds both dis- 

 tinct canals and lacunar spaces (pi. 22, figs. 13, 14), which radiate from the point of connec- 

 tion of the pad with the body, and these together with the arrangement of the cells give 

 the radiate stucture characteristic of the fresh gill. The canals are generally circular in 

 section and pursue irregular courses throughout the cell substance, while the spaces are 

 developed by the separation of adjacent cell walls and are irregular in outline and occur 

 at varying distances from each other. The gill pad is essentially a single-layered sac, with 

 a much constricted neck, evaginated from the pleural region of the abdomen- The pro- 

 truding organ is flattened against the body of the embryo and by this means the cells are 

 rendered spindle-shaped. The nucleus of each cell lies in that part of its cell which is farthest 

 from the constriction of the organ. The cell wall gradually tapers to a point and ends near 



