178 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
in character and the organs consequently differ in structure according 
to the impressions they are to receive. 
With very few exceptions the characteristic portions of the organs, 
the sensory cells, arise from the ectoderm, but accessory parts, chiefly 
of mesodermal origin, may be so abundant as to form the bulk of the 
organ. In some cases the organs may remain in connexion with the 
surface of the body (the parent ectoderm) throughout life, but frequently 
they sink to a deeper position and become surrounded with a protective 
sense capsule, while those connected with the sympathetic system may 
be scattered throughout almost the entire body. 
Fic. 175.—Free nerve termina- Fic. 176.—Sensory cells, after Fiirbringer. 
tions in the skin of Salamandra, a, crista cell of ear; b, rod cell of eye; c, ol- 
freely after Retzius. factory cell. 
The recipient structures may be of two kinds. In the one (fg. 
175) the ends of the nerve receive the impressions from without, often 
aided by various accessory structures. In the other there are specialized 
sense cells (fig. 176), the peripheral ends of which bear different kinds 
of cuticular percipient parts—hairs, bristles, rods, cones, etc.—while 
the basal ends of the cells are connected with the terminations of nerve 
cells which act as the conducting elements. The distinction between 
the two is one of convenience rather than one of physiological or mor- 
phological importance, for the ‘nerves’ of the first are in reality but 
the prolongations of sensory cells. 
Nerve-end Apparatus. 
In many cases—skin, alimentary tract, muscles, etc_—the ends of 
the sensory nerves lose their medullary sheath and break up into fine 
fibrilla which terminate, without special accessory structures, among the 
cells of the tissue to which they are distributed (free nerve termina- 
tions). On the other hand, there are numerous end organs, espe- 
