The Nervous System of Amblystoma. 471 



that other structures outside of the neural plate may have a 

 part in the formation of the brain. 



The position taken by Patten is that the vertebrate brain is 

 built up of various elements, some of which lie morphologically 

 outside of the primitive neural axis. This hypothesis has been 

 very suggestive. Some attempt has been made in this paper 

 to test it by tracing as far back as possible the very beginning 

 of Lhe various parts of the brain, and by discriminating very 

 carefully between structures that belong primarily to the neural 

 plate and those that do not belong to it. It has already been 

 shown that the germinal depressions are probably not "neural" 

 grooves. The history of the neural crest has shown that it 

 also is evidently not an integral part of the neural plate since it 

 makes its appearance only after the plate is well marked off, 

 since it is separated from the plate by a distinct groove and 

 since it is not segmented like the plate. It now remains to 

 discuss in this connection the development of two still more 

 important structures, the lateral eyes and the optic lobes. 



The first appearance of the lateral eyes has already been 

 described as a pair of pigment spots lying just lateral to the 

 anterior part of the neural plate in the peripheral groove. As 

 the neural crests are raised up to form the neural tube the retinal 

 spots come to lie in the lateral walls of the primary fore-brain 

 vesicle (rs, fig. 8, A). The walls of the brain immediately begin 

 to bulge out to form the optic vesicle (e, fig. 8, B), the direc- 

 tion of growth being backward as well as outward, as may be seen 

 by comparing the figures of this stage with those of the next 

 stage. The optic vesicle is comparatively large, as in most ver- 

 tebrates, and seems to form at this stage the whole lateral wall 

 of the fore-brain vesicle. 



The histology and histogenesis of the retinal spot in Ambly- 

 stoma and other amphibians, Rana palustris for example, has 

 been described in detail by Eycleshymer ('93). He found that 

 in Amblystoma the pigment in the early stages is not so well 

 developed as in Rana palustris, neither do the retinal cells show 

 the early differentiation into the characteristic columnar form. 

 My own observations confirm Eycleshymer's work. It seems 



