'7'74 t>r. J. BEARD, 



Jacobson's organ; for Jacobson did not discover it, and it is not 

 homologous with the organ he did detect. 



It may here be noted, and Prof. Wiedersheim agrees, that the 

 division of the olfactory nerve into two branches in the Gymnophiona 

 cannot be regarded as corresponding at all to the division of the 

 branches of a cranial nerve to each side of a gill-cleft as Wieders- 

 heim (33) and Marshall (33) formerly considered. It is only a di- 

 vision of the olfactory nerve into two portions comparable to, though 

 hardly homologous with, the divisions of the olfactory nerve in lizards 

 and snakes into two branches, one to the nose proper and the other 

 to Jacobson's organ. 



This brings us to the disputed question of the quondam existence 

 of prae-oral clefts, a matter which cannot be discussed here in detail. 



Dohrn and Marshall (39 and 52), as is well known, have endeav- 

 oured to establish the claims of the nose to rank as a modified gill- 

 cleft. In a former paper I tried to show that this view was untenable. 

 Regarding that attempt a good deal of misunderstanding exists, and 

 it will be well to consider once more the true morphological nature 

 of the nose in the light of recent work. In my opinion there does 

 not exist a particle of evidence to show that the nose is a modified 

 gill-cleft, and at present, the evidence that a gill -cleft formerly 

 existed on the haemal side of the nose is to many zoologists not very 

 convincing. 



There is nothing in the development of the nose per se to sug- 

 gest a gill cleft, on the contrary, the comparison of the olfactory 

 nerve and organ with the sensory portion of a cranial branchial nerve, 

 its complicated ganglia and sensory epithelium is now complete. 

 There is no evidence of branchial musculature and cartilages, of motor 

 roots of the olfactory nerve, still less of the existence of an actual 

 branchial cleft. 



Like a typical branchial nerve, the olfactory nerve develops ele- 

 ments from two sources : from the epiblast just outside the foundation of 

 the central nervous system, and from the special neuro-epithelium. The 

 latter, like that of the facialis or vagus, grows in length by increase 

 within itself, and in many cases, Poli/pterus (Waldschmidt), some Te- 

 leosteans, Amphibians and Reptiles, again like the neuro-epithelia of 

 facialis or vagus, it later on in development divides up into a number 

 of smell-buds, comparable exactly to the sense organs of the la- 

 teral line. 



