120 



In Acanthias, they form a series of cup -like depressions on the 

 neural plate extending back of the eye-vesicles. They make their first 

 appearance while the cephalic plate is broadly expanded, and become 

 better differentiated as the brain walls grow upwards. "The optic 

 vesicles are formed first and when, at a very little later stage, the 

 others arise behind them, it appears as if the process of eye-formation 

 were repeating itself serially," Sections show that there are not less 

 than 8 pairs of these "accessory optic vesicles" in Acanthias. They 

 not only arise in a similar way, but structurally resemble the optic 

 vesicles. In cross and longitudinal sections, the cells in the accessory 

 eye-pits are similar to those in the optic vesicles. They are, likewise, 

 in Acanthias very transitory and disappear while the neural groove 

 is closing. 



There are structural resemblances, of a broad and general cha- 

 racter, between these accessory structures in Acanthias and the chick. 

 They arise in both cases in close connection with the optic vesicles, 

 they are serially arranged behind the former and resemble them in 

 structure and method of formation; the fact that they are smaller is 

 about the only distinguishing feature of a structural kind, and their 

 brief history of rise and decline in the two animals is similar. 



These observations bear directly on certain questions concerning 

 the vertebrate eyes. If the views expressed above are correct, they 

 place those sense-organ in the category of segmental sense-organs, 

 and the basis on which the conclusion is now made to rest, is their 

 serial relation with other patches of neural epithelium. The view 

 that the vertebrate eyes are segmental is by no means new, but, there 

 has been great dissent to accepting it, by those morphologists wlio 

 hold there is such a fundamental difference between cerebral and peri- 

 pheral sense-organs, that the evidence of the segmental nature of the 

 one kind, cannot be used in support of the segmental nature of the 

 other. There has been but little direct evidence to satisfy this re- 

 striction, as far as the eyes are concerned, but the facts now presented, 

 as far as they go, meet the requirement. 



The segmental nature of annelid eyes has been demonstrated by 

 Whitman, and he has also argued for the same condition of the 

 vertebrate eye. In 1889, he said^): "Take what are now incontestable 

 facts in the phylogeny of annelid and arthropod sense -organs, and 

 add to them the evidences in favor of the common derivation of the 

 vertebrate organs of special sense, and is it not enough to awaken a 



1) Journ. Morph., Vol. II, April ISSÜ, p. 595. 



