162 COMPARATIVE ANATOMY. 



Their extraordinary richness in blood-vessels, which is seen 

 throughout life, points to the important function of these organs ; 

 but it is impossible to say at present what this function is. 



BIBLIOGRAPHY. 



BALFOUR, F. M. Elasmobranch Fishes. London, 1878. 



BRAUN, M. Ui-brr Bau und Entwickl. d. Nebennieren bei Ecptilien. Arb. d. zool. 



Inst. zu JFilrzburg, Bd. V. 

 GOTTSCHAU, M. Ucbcr Nebennieren der Sdugethiere spec, uber die des Menschen. 



H'i/rzb. phys. mcd. Gesellsch. 1882. Structur und embr. Entwicklung dtr 



Nebennieren bei Sdugcthiercn. Arch. f. Anat. und Physiol. 1883. 

 JANOSIK. Bemerkungen ub. die Entwick. der Nebennieren. Archiv f. mikr. Anat. 



1883. 

 MITSUKURI. On the Development of the Suprarenal Bodies in Mammalia, Quart. 



Journ. of Micros. Science, 1882. 

 ONODI, A. D. Ueber die Entwick. des sympatischcn Ncrvensystems. Archiv f. mikr. 



Anat. Bd. XXVI. Heft 1. 

 WELDOX, "VV. F. R. On the Head-Kidney of Bdellostoma, with a Suggestion as to the 



Origin of the Suprarenal Bodies. Quart. Journ. of Micros. Science, 1884. Cn 



the Suprarenal Bodies of Verte'brala. Quart. Journ. of Micros. Science, 1885. 



III. SENSORY ORGANS. 



The specific sensory end-organs originate, like the nervous 

 system in general, from the epiblast. The peripheral terminations 

 of the sensory nerves are thus always to be found in cells of 

 epithelial origin, while mesoblastic elements (as investments, for 

 instance) are secondarily added to them. 



The individual sense-organs, e.g. those of sight, smell, taste, and 

 hearing, are to be regarded as secondary differentiations 

 of a diffused sense, as will be mentioned later on. This is 

 not only indicated in many Invertebrates, but numerous facts in 

 the development of Fishes and Amphibia and in the structure of 

 Amphioxus point to the same conclusion. 



From the Cyclostomi onwards this differentiation has already 

 taken place, and we find in them and in all Vertebrates above 

 them, that the olfactory, optic, and auditory organs are always 

 closely connected with the head. They are enclosed within 

 certain cavities and hollows of the skull (sense-capsules), and 

 thus differ somewhat from the second main group of sensory organs, 

 which are the means whereby the sensations of taste and temper- 

 ature, as well as other sensory impressions, are appreciated. These, 

 for the greater part, extend over the whole surface of 

 the body, and thus have a diffused distribution; more- 

 over, they remain throughout life near the ectoderm, 

 from which they originate. 



In the higher organs of sense two kinds of cells are to be 

 distinguished, although they are genetically identical with one 

 another. The first of these are the proper rod -shaped sensory 



