330 DR. A. MILNES MARSHALL. 



the arrangement and distribution of the blood-vessels, and 

 in the characters of their surface epithelium. 



h. General considerations. — Hitherto we have been con- 

 cerned simply with matters of observation ; though, indeed, 

 1 have not attempted to give a complete account of the 

 development of the olfactory organ, but have limited my 

 description to certain developmental features, in which it 

 strongly resembles the visceral clefts ; still I have dealt 

 simply with facts, or what I believe to be facts. I propose 

 now to consider the subject from a more theoretical point of 

 view. 



In the first place I would submit that the very close 

 resemblance as to form, structure, general relations, time of 

 appearance, &c., existing between the olfactory organ and 

 the gill clefts, whether these be considered as wholes or in 

 their separate parts, is sufficient to raise a strong probability 

 that they are homologous structures. 



This probability is strengthened by the complete absence 

 of similar structures in any other part of the body at any 

 period of development. Not only do the Schneiderian folds 

 and the gills appear at the same time and agree completely 

 in structure, but in no other part of the body do similar 

 structures occur, either at this or any other period. 



Again, this probability gains very material support from 

 the conclusion arrived at in the first part of this paper, viz. 

 that the olfactory nerve is a segmental nerve ; for we hav« 

 seen that one of the most important diagnostic characters of 

 a segmental nerve is its distribution to the two sides of a vis- 

 ceral cleft, and, since the olfactory nerve is distributed to the 

 olfactory organ, and to that alone, if there be a visceral cleft 

 with which it is in relation, the olfactory organ must be 

 that cleft. 



The conclusions, then, to which I have been led concerning 

 the morphology of the vertebrate olfactory organ are — that 

 the olfactory organ is the most anterior visceral cleft ; that 

 the olfactory nerve is the segmental nerve supplying the tioo 

 sides of that cleft i?i a manner precisely similar to that in 

 which the hinder clefts are supplied hij their respective nerves ; 

 and that the Schneiderian folds are homologues of gills. 



The suggestion that the nasal organs are gill clefts was 

 originally made by Dr. Dohrn, in his essay on the origin of 

 vertebrates. In discussing the question whether the pair of 

 gill clefts, which by their median fusion formed the verte- 

 brate mouth, was the most anterior pair, Dr. Dohrn says : 

 " Aber auch betreflfs der vorderea Kiemenspalten ist noch 



