Morphological Studies. 767 



The skeletal portions of the nose and Jacobson'» organ have 

 been well-investigated by others, especially by Born (4, 5, 6), and so 

 we can here leave them unnoticed, confining our attention to the 

 changes which occur in the sensory epithelium proper. 



In the preceding pages I have described how in Elasmobranchs 

 the olfactory epithelium gives rise to a certain number of ganglion 

 cells, which I call the lateral ganglion. 



The same thing takes place in the lizard, and to a far greater 

 degree in the snake, in both the nose-proper and Jacobson's organ. 

 As the changes are most obvious in the Ophidia, I shall describe 

 what I have observed in several snake embryos ( Tropidonotus natrix). 

 At the same time I must mention , and hope to demonstrate, that 

 the structures described below are also to be found in the lizard, but 

 in a more disguised form. 



Fig. 10, a section of an Anguis embryo, shows that the epi- 

 thelium of both Jacobson's organ {Jo), and of the nose (oe), has 

 acquired a considerable thickness. An identical stage of T. natrix 

 could have been represented. 



Leaving, for a time, the nose out of question, we may note that 

 the wall of Jacobson's organ is composed 1) of an epithelium lining the 

 cavity of the organ, and this epithelium will give rise to the sense 

 cells ; 2) internal to this several rows of „nuclei" or cells which are 

 of a ganglionic character. 



In this section the olfactory nerve has already acquired its per- 

 manent connection with the brain, and, indeed, the olfactory lobe is 

 fairly developed. The nerve is made up of two branches, a neural 

 or dorsal one to the olfactory organ proper, and a haemal or ven- 

 tral one to the organ of Jacobson. The latter is a very thick branch 

 and contains numerous nuclei in its course. 



The deeper layers of the sensory epithelium, i. e. the ganglionic 

 layers, now begin to proliferate ganglionic elements at regular intervals 

 into the mucosa. The epithelium throws out, as it were, a number of 

 bud-like processes (fig. 6). We shall soon see what this means. 



Fig. 6 is a vertical section of Jacobson's organ in a snake em- 

 bryo. The organ has now its characteristic adult form. The sensory 

 epithelium is very thick, and its outer side, which is connected with 

 the large branch of the olfactory nerve, forms a number of very 

 symmetrical and striking projections. One of these nobs and its portion 

 of the sensory epithelium is represented in fig. 5. On examining 

 these projections more closely we see that the fibres of the olfactory 



