ATTRACTIVE AND REPULSIVE ODORS 407 



course. Besides these tubules, some plates of what appears to be chitin 

 pass down between some of the cells. 



The next two layers are cellular. That lying next to the cuticle is 

 very thin and appears as almost a line except where its contained nuclei 

 give it a greater breadth. It follows the sinuosities of the cuticle and its 

 nuclei are placed most often in the upper curvatures. The tubules just 

 spoken of penetrate this layer through or between its cells. It seems 

 more probable that they pass through these cells. 



Immediately beneath this layer is found the layer of secreting cells. 

 They are thick and heavy, being roughly twelve to fifteen times the thick- 

 ness of the chitinous layer and five to seven times its thickness in width. 

 Some of the narrower ones are not secreting, and possibly may develop 

 later into gland-cells. Those that are secretory show a very large, clear 

 vacuole slightly above their middle height. This vacuole is marked 

 off from the heavy, granular cytoplasm by a distinct membrane. It is 

 evidently homologous with the clear cytoplasmic space described in the 

 odoriferous cell in Forficula, but is a higher specialization. That in 

 Forficula had no bounding membrane besides other differences to be 

 noticed in the drawings and descriptions. 



The cuticular tube, mentioned above as coming down through the 

 hypodermal cells from the cuticle, enters into the top of a secretory cell 

 and penetrates to the vacuole which it enters. Here it ends in a round 

 knob and it also apparently ends blindly, for its lumen comes to a blind 

 point in the center of the knob, and there is apparently no further 

 opening. 



Fine fibrils reach from all points of the vacuolar wall to this knob 

 which is placed near its distal wall. They make a beautiful radial pic- 

 ture which is well seen in three places in Figure 368. 



The cuticular tube obviously serves the purpose of conducting away 

 the secretion. The fact that a simple tube can and does perform this 

 function in the earwig adds to the mystery of why a round, heavy 

 knob should be placed over the end of the same structure in Belostoma. 

 That it does carry out the secretion in this animal seems more than in- 

 dicated by the collected granular material heaped up in the cuticle flexure 

 into which it opens. 



A word must be said here regarding the origin of the secretory layer 

 in this gland. If the thin, hypodermal layer directly under the cuticle 

 represents a perfect layer and all of the ectodermal cells in the gland, 

 then the secretory cells are mesodermal in origin and the penetration of 

 then- body by the cuticular tubule is a secondary relation. On the other 

 hand, the presence of this tubule and the fact that the cells are secretory 

 lead the writers to believe that the secretory cells are derived in the 

 embryo from the hypodermal layer and have acquired their proximal 



