30 riiK.SlDE>'T's ADDKEiiS. 



size early in development, and afterwards undergoing retrogres- 

 sion. At an early stage we only need to conceive a slit to be 

 made in the nerve tube at the bottom of the fossa rhomboidea, 

 in order to furnish a suitable passage into the alimentary canal. 

 His fii'st reason for regarding the vertebrate mouth as a modem 

 structure is that it arises so extraordinarily late in development. 

 The embryonic body is almost completely fi'amed, all the great 

 systems arc established, the circulation is in active operation, 

 while as yet there is no mouth. Again, the mouth does not 

 arise in the position in which it permanently remains in the 

 great majority. It undergoes considerable shifting forwards. 

 Only in the Selachians and Ganoids does it retain its primitive 

 situation. Moreover, the study of development is steadily tend- 

 ing to establish the idea that the mouth of vertebrates is homo- 

 dynamous with the gill-clefts. It is limited, like them, by a pair 

 of arches, lies just in front of the first pair of gill clefts, arises 

 simultaneously with them in the embryo, aild opens into the ali- 

 mentary canal. A glance at the ventnd surface of a Ray shows 

 the likeness of the mouth to a pair of coalesced gill-clefts. Con- 

 sequently, it becomes probable that the present mouth opening 

 once existed functionally as a gill-cleft, that at a certain period 

 in the ascending development both the old and the new mouths 

 supplied nourishment, that the latter gained the predominance, 

 and that finally the old mouth became aborted. 



The next problem attacked is the origin of the gill-clefts. A 

 very elaborate account is given of the supposed process by which 

 the external gills and segmental organs of Annelids were meta- 

 morphosed into the gills and the gill-clefts of vertebrates and the 

 skeletal elements connected w4th them. The great difiiculty 

 whicli Dohrn confesses in this matter is the connection of the 

 inner extremities of the segmental organs with the wall of the 

 alimentary canal. But if this be granted, it is comparatively 

 easy to understand how the shortening and the widening of the 

 segmental organs might give rise to gill-cavities such as those of 

 the Selacliians. 



It follows, from llie view of the origin of \ertebrates thus ox- 

 pounded, tluit Ami)hioxu8 loses much of its interest, for there is 



