

THE SENSE ORGANS. 387 



The outer or cortical portion of the supra-renal body is 

 developed from a mass of mesoblast cells which appears about the 

 twelfth day, lying in front of the kidney, and between the aorta 

 and the root of the mesentery. 



The medullary portion of the supra-renal body is formed from 

 a column of cells, which grows out from the longitudinal sym- 

 pathetic cord about the fifteenth day, and makes its way into the 

 mesoblastic mass which gives rise to the cortical layer. This 

 mode of development agrees with what is known as to the for- 

 mation of the supra-renal bodies in other Vertebrates, but leaves 

 the real morphological meaning of these curious structures still 

 undecided. 



THE DEVELOPMENT OF THE SENSE ORGANS. 



1 . The Nose. 



There is very little of special interest about the olfactory 

 organ of the rabbit, which resembles in most features of its 

 development that of the chick. A special diverticulum arises 

 at an early stage from each olfactory sac, which acquires a 

 separate opening into the mouth, through the naso-palatine 

 canal, and becomes the organ of Jacobson. 



2. The Eye. 



The general history of development of the eye of the rabbit 

 is very similar to that of the chick ; the formation of the optic 

 vesicles as outgrowths of the fore-brain, the doubling up of the 

 vesicles to form the optic cups, the pitting-in of the surface 

 epiblast to form the lens, and the subsequent fate of the several 

 parts being essentially the same in the two cases. One of the 

 most marked points of difference is the much smaller size of the 

 eye in the rabbit (cf. Figs. 115 and 148). 



The optic vesicles arise as lateral outgrowths of the fore- 

 brain, at a very early stage. Before the end of the ninth day, 

 i.e. before the fore-brain is closed dorsally, the optic vesicles 

 (Fig. 145) are already conspicuous structures. The vesicles 

 soon become constricted at their origins from the fore-brain, the 

 constricted portions giving rise to the optic stalks. As the con- 

 strictions proceed from above downwards, the optic stalks remain 



c c 2 



