768 ï^r. J. BEARD, 



nerve end in the little nobs of cells , and that each of these little 

 nobs is neither more nor less than a collection of ganglion cells 

 (fig. 6), formed from the neuroepithehum, projecting at regular inter- 

 vals from it, and forming a mosaic-like pattern. The nobs of cells 

 are very characteristic of Jacobson's organ in snakes, and there are 

 from 150 to 200 of them in each organ. In a slightly later stage, in 

 which the cartilage is beginning to be formed (fig. 2), the organ has 

 become more like that of the adult. 



The sensory epithelium is now made up of the following consti- 

 tuents: (1), an inner layer of sense cells (sfc), beyond these (2), 

 several layers of elongated „nuclei" ; more outwards (3) , a fairly 

 thick layer of more rounded „nuclei"^), and, outside of all (4), the 

 little nobs of ganglion cells. These latter are still more marked than 

 in the proceding stage, and the fibres of a large branch of the olfac- 

 tory nerve end in them. They possess now somewhat of a pear-shaped 

 appearance, and I shall speak of them , for reasons to })e afterwards 

 developed, as the smell-bud ganglia. Morphologically they are 

 the homologues of lateral ganglia, for like them they owe their 

 origin to the neuro-epithelium , which in its turn is homologous with 

 the sensory epithelium of a segmental lateral sense organ. 



A number of these smell-bud ganglia, and the columns to which 

 they give rise, are drawn, as seen in the section just described, in 

 fig. 4. 



The next stage to be considered is much more advanced, and 

 the cartilages are all fully formed. It is represented in fig. 1, a 

 longitudinal vertical section of an advanced Tropidonotus embryo of 

 10 to 12 centimetres in length. 



The section passes through one olfactory lobe {ol I) and the 

 course of its olfactory nerve {ol). The nerve is divided into the two 

 branches described above, and the section cuts the Jacobson's organ 

 brauch at its thickest portion. Part of the branch to the nose proper 

 is also cut, and the plane of section has passed through the inner 

 wall of the nasal cavity. 



The wall of the duct of Jacobson's organ, (J o d) is also seen 



1) The histologist usually speaks of such structures as these as 



„nuclei". The morphologist , however , must assign a true ganglionic 



cellular nature to them, no matter how small the body of the cell may 

 be, here they are no doubt ganglionic. 



