34 Scientific Proceedings. Royal Dublin Society. 



myotomes, it follows that Marshall's theory as to the division of 

 the mesoderm head-segments into ventral and dorsal portions falls 

 to the ground. Only ventral mesoderm formations remain. 



In considering the phylogenetic history of the eye, it is of the 

 utmost importance that the muscles moving it should belong to the 

 same category, as the muscles of the visceral arches, viz. the bran- 

 chial and mandibular muscles. 



The great revolution produced by the removal of the mouth 

 in the ancestral vertebrate to the dorsal surface (the ventral in 

 living forms) must likewise have had a considerable effect on the 

 structure of the eye. We must suppose, says Dohrn, that the 

 medullary canal began to close, and that the eye, . on account of 

 the above-mentioned great changes, was somewhat removed from 

 its former position. In thus changing its position, it approached 

 in all probability the gill-clefts surrounding the new mouth. We 

 may further suppose that rays of light now fell through one or more 

 of these clefts into the eye, one of which may already have lost 

 its function, and had therefore discontinued its endodermal con- 

 nexion. The ectodermal invagination or pit of this gill-cleft is 

 then supposed to have formed the lens of the eye, whose peculiar 

 mode of formation would thus be explained. This hypothesis is 

 strengthened by the occurrence of the choroid gland in the teleostean 

 eye, which receives its blood supply from the pseudo -branchial 

 vein. The choroid gland may be looked upon as a rest of a 

 former gill, which, having lost its ecto and endodermal connexion, 

 is merely represented by a network of blood-vessels. 



The arteria opthalmica is probably the artery which supplied 

 this ancestral gill with blood. This hypothesis likewise explains 

 the meaning of the pecten in the eye of ^reptiles and birds, as well 

 as the embryonic vessels in the mammalian eye, and a number of 

 other anomalous formations. All of them may be regarded as 

 remnants of the gill now represented by the lens. 



Dohrn then leaves the consideration of the muscles and nerves 

 supplying the eye for the present, in order to get additional support 

 from other facts. 



In a previous study he enunciated the view, that the tail of the 

 vertebrate animals represented mostly dorsal parts of its original 

 composition, and that the head, on the other hand, exhibited 

 principally ventral parts, the brain being the only remnant of the 



