754 ï>r. J. BEARD, 



One criticism of Prof. Gegenbaur's can be here at once admit- 

 ted, for it is a just one, viz., that the figures in my former contri- 

 bution (36) were not as clear or as well drawn as ought to have been 

 the case, and if an error lay in this fact, an attempt is here made 

 to remedy it. If my conclusions as to the morphology of the nose 

 are once more persistently urged in this paper, is is not because I see 

 fit to uphold my views at any price, but because the renewed series 

 of researches here recorded, when considered in the light of recent 

 work (37) of mine on the peripheral nervous system, lead, as I think, 

 irresistibly to the conclusion that the nose, like the ear, is part of 

 one system of sense organs; a system which Vertebrates have 

 inherited from their Annelidan ancestors. This statement is meant 

 to be a modified support of Dr. Eisig's valuable comparisons (41, 

 42) of the lateral sense organs of Annelida and of Vertebrata. In a 

 future Study the conditions under which this homology can be accept- 

 ed will be clearly laid down. 



The power to evolve new sense organs from indifferent found- 

 ations must, in my opinion, be absolutely refused to Vertebrates. A 

 Vertebrate, now or at any time in its past history as a Vertebrate, 

 can only form sense organs by further differentiations of the segmen- 

 tally arranged sense organs, inherited from its Annelidan ancestors, 

 and from its eyes which have nothing to do with the latter sense 

 organs. The eyes certainly do not „swim in the same boat" as the 

 nose and ear, as Dr. Hill expresses it (46, p. 5). 



To refer to some of these supposed differentiations. It is im- 

 possible to understand the development of the parietal eye except as 

 a differentiation of a part of the sensory tissue of the paired eyes, 

 and, as insisted on in the first of these Studies, it is not difficult 

 to make out the evolution of the parietal eye in this manner. 



If the position taken up for the parietal eye is partly hypotheti- 

 cal, the matter is very different when the evolution of Jacobson's 

 organ of Lacertilia and Ophidia is considered. Its actual development 

 is described in the following pages, and it will then be demonstrated 

 that what is practically a new sense organ is merely derived from a 

 portion of an old sense organ, the nose. 



That which has happened in the case of Jacobson's organ in La- 

 certilia and Ophidia has taken place in all Vertebrates (except the 

 outcast Amphioxus) to a greater or less extent in the ear. There 

 from a low sense organ, homologous with a lateral or branchial sense 



