l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. "J"] 



the latter is made in the immediate neighhorhood of the cell. From 

 his observations, made on specimens stained by the Golgi method, 

 Vogel claims that the antennal nerve of insects is not to be likened 

 to the olfactory nerve of vertebrates, and that the innervated cell 

 of an insect sense organ becomes secondarily a sense cell by union 

 with a sensory nerve fiber. 



It still, therefore, appears to be true, as Berlese (1909) has 

 said, that those v^riters " who had hoped for a demonstration of 

 the hypodermal cells themselves becoming ganglion cells, have awaited 

 in vain that this should be proved." Yet, Vogel's conclusion that 

 there must be found in the deutocerebrum a ganglionic center from 

 which the sensory antennal nerves take their origin, has also not 

 been substantiated, for the elaborate studies of vom Rath, Kenyon, 

 Haller, and Jonescu have failed to reveal anywhere within the 

 central ganglia of insects the cytons of the sensory fibers. Perhaps 

 they lie somewhere between the extreme periphery and the nerve 

 centers. The subject of the origin of the sensory nerves in insects 

 or other invertebrates is one on which the embryologists are strangely 

 silent, and until further investigations shall give us more light upon 

 it, we cannot reconcile the two apparently contradictory sets of 

 observations. Either the claims from one side or the other are 

 incorrect, or there is some undiscovered source of the sensory axons, 

 possibly corresponding with the neural crests of the vertebrate 

 embryo. 



II. THE PERIPHERAL ENDINGS OF THE SENSORY NERVES 



Some nerve trunks consist entirely either of motor or of sen- 

 sory fibers ; but, for the most part, the two kinds of fibers, after 

 leaving the central ganglia, are bound together in the same nerve 

 trunks. Toward the periphery, however, the two sets of fibers 

 again separate, and individual axons proceed to their own desti- 

 nations. 



The peripheral ends of the sensory fibers, as we have already 

 seen, end in bipolar or multipolar cells, the distal processes of which 

 either go direct to specific ectodermal sense organs, or they break 

 up into fine branches beneath the hypodermis and on the wall and 

 muscles of the alimentary canal. The bipolar cells with unbranched 

 distal processes going to the external sense organs are distinguished, 

 according to the classification of Zawarzin (1912 a), as sensory 

 cells of Type I ; the bipolar and multipolar cells, with branching 

 terminals or dendrons, as sensory cells of Type II. Since the former 

 belong to the cellular complexes of the sense organs they will be 



