The Nervous System of Amblystoma. 435 



cular area." Whitman has proposed a theory which to-day 

 attracts more attention than Lankester's theory, although it 

 will be seen that the two theories do not necessarily conflict. 

 His hypothesis is as follows: ''The medullary plate of the verte- 

 brate is undoubtedly an enormous extension of the ancestral 

 invertebrate plate. Sense organs lying originally outside the 

 neural plate have probably in consequence of this extension in 

 width, been brought within the medullary area." 



Eycleshymer ('94) continued Whitman's work on the origin 

 of the eyes of the Amphibia. Of the three forms which he studied, 

 Necturus, Amblystoma, Rana palustris, he found the last named 

 the most satisfactory for his purpose on account of the deeper 

 pigment of the eye spots and their greater histological differen- 

 tiation. He studied very fully the histology of the pigmented 

 areas, but he leaves it a little in doubt from his plates and descrip- 

 tions whether those areas are actually on the neural plate or 

 between the plate and the crests, a very important point in test- 

 ing Whitman's theory that the ancestral invertebrate plate has 

 widened out so far as to irfclude the region of the lateral eye. 

 His conclusion in favor of the hypothesis that the vertebrate 

 eye must have been located originally within the brain, and his 

 statement in support of this theory that "this was precisely 

 the case in Rana palustris," must be taken to mean that the 

 original brain comprised both neural plate and neural crests. 



In criticism of this theory of the relation of the eyes to the 

 primitive brain it may be suggested that the neural plate proper 

 represents the central nervous system of the ancestors of the 

 vertebrates, and that the anlage of the vertebrate lateral eyes 

 are located between the neural plate and the neural crests. If 

 this be true it is probable that the medullary plate has not under- 

 gone such an enormous extension as Whitman supposed and that 

 the ancestral vertebrate eyes were not located on or in the brain, 

 as Lankester and Eycleshymer supposed, but on the margin 

 of the neural axis, as in arthropods, and that by a folding over 

 of the non-nervous sides and roof of the brain they became in 

 included in the neural canal in a similar way to that described 

 by Patten for the arachnids (Patten, '89). Such a view as this 

 obviates the necessity of assuming that the nervous tissue in- 



