NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 



39 



structures, we are not warranted iri assuming that a cuticular 

 process ever grows inward except as an ingrowth from the exterior 

 surrounded by a hypodermal matrix. A muscle " tendon " serves as 

 a good example of a structure produced in this way. As shown by 

 Janet (1907), the long tendon-like stalk to which some insect muscles 

 are attached is produced from a single hypodermal cell. The tendon 

 cell (fig. 16 C, TndCl) elongates and its interior secretes a chitinous 

 continuation (I'nd) from the outlying cuticula (Ct), which finally 

 (B) occupies the entire length of the mature tendon cell and forms 

 at its inner end a funnel-shaped cup holding the end of the muscle 



Fig. 16. — Theoretical suggestions of the morphology of the sense rod. 



A, sense rod {SR) of a campaniform organ of Dytiscus, as illus- 

 trated by Hochreuther (1912), attached to cuticular part of organ 

 {Ct) and ensheathing the end of distal process (d) of a sense celh 

 B, structure of a muscle tendon as illustrated by Janet (1907), the 

 tendon (Tnd) is an intracellular product continuous with the cuticula 

 (CO- C, early stages in formation of muscle tendons (Janet). D, 

 theoretical relation of a sense rod (SR) to distal process (d) of a 

 sense cell suggested by comparison with a muscle tendon (B). E, a one- 

 celled gland with intracellular duct. 



(Aid). The tendon is probably to be regarded, theoretically, as a 

 hollow ingrowth of the cuticula, corresponding structurally with the 

 duct of a one-celled gland (fig. 16 E, Dct). The important feature 

 to be noted in both these structures, however, is that the hypodermal 

 matrix surrounds the cuticular ingrowth. 



Applying this principle of the known method of growth of internal 

 cuticular organs to the sense rods, we must suspect that each rod 

 is formed within the distal end of the sense cell process, as shown 

 diagrammatically at D of figure 16, rather than on the outer surface 

 of the process. If produced in the second manner, the rod could not 

 be renewed after a molt and reestablish its connection with the outer 

 cuticula, especially where the end of the sense cell is retracted from 

 the latter. The parts in question are so minute that the actual facts 



