Morphological Studies. 765 



These are the first differentiations of the nose proper and Ja- 

 cobson'« organ. In a vertical section, such as the one depicted in 

 fig. 27, it may be remarked that the one depression is directed neu- 

 ralwards, and in transverse section outwards (fig. 29), while the other 

 depression grows backwards (in longitudinal section, fig. 28), and in- 

 wards (in transverse section, figs. 29 and 30). These depressions both 

 get deeper and deeper (figs. 27 and 28), and become more and more 

 constricted ofi" from each other. The changes which have taken place 

 during all this time have been brought about by the growth and further 

 differentiation of the simple piece of ueuroepithelium figured in 

 fig. 12. 



This is obvious from a glance at fig. 23, which represents the 

 nose proper, oe, and Jacobson's organ, «/o, in a longitudinal section 

 of a fairly advanced embryo of Anguis fragilis. In this figure the 

 limits of the ueuroepithelium are very easily determined, and it is 

 seen that the sensory epithelium does not pass gradually but abruptly 

 over into the ordinary epiblast. 



The stages from this point onwards, that is from figs. 29 and 30, 

 seem to have all been seen by Born (4) and Fleischer (8), and I 

 therefore venture to quote Fleischer's description in full (8, pp. 10 

 to 11): „In the earliest stages at ray disposal a diverticulum, such 

 as we have seen in the Mammalia, can be observed. But here it is 

 much larger than in the latter. It is covered by an epithelium, and 

 occupies almost the half of the entire olfactory depression. From its 

 situation we may homologise this diverticulum with the lower cul-de- 

 sac observed in the Anura, and consider the rest of the nasal cavity 

 as composed of an upper and middle cul-de-sac. Just as in the 

 Anura, we find this diverticulum in some sections in communication 

 with the nasal cavity, in others separated off from it. 



In the further course of development it passes further downwards, 

 and in doing so gets turned about its axis, so that the canal, by 

 means of which it was still connected with the nasal cavity, comes 

 now to open into the mouth cavity. Even in very early stages it 

 (the diverticulum or Jacobson's organ) becomes covered by a carti- 

 lage (see the figures 29 and 30 of this paper), which, similarly to 

 that cartilage we noticed in the Mammalia, projects forwards from the 

 nasal septum in a mushroom-form towards Jacobson's organ. In doing 

 so it pushes the epithelium of the lower wall before it, so that the 

 lumen of the organ acquires a semi-lunar form" (see fig. 9). 



