The Nervous System of Amblystoma. 473 



it is seen that the optic lobes have become thinner and also a 

 little longer. The cavity, too, has become larger. This 

 enlargement of the optic lobes is seen in surface views of fig. 9. 

 Sections E, F, G are longitudinal sections cut in a plane at right 

 angles to sections B and C. Their position is shown by the line 

 in the diagram at D. Section E shows the optic lobe extending 

 posteriorly from the optic vesicle very much as in B. In section 

 F the lobe has become slightly divided into two parts and in 

 section G it is very clearly divided (mbl, mb2). This last sec- 

 tion shows also how closely the cerebellar crest is related to the 

 general neural crest as has already been shown in fig. 9. 



The histology of this region presents no very distinctive fea- 

 tures as the histology of the optic vescile showed none. Section 

 C, fig. 11, shows the cells of the peripheral groove (pg), the region 

 in which the optic lobes arise. The cells present the same appear- 

 ance as those of the neural plate but they differ strikingly from 

 the cubical cells of the adjacent neural crest. There is slightly 

 more pigment in the peripheral groove than in the plate. Sec- 

 tion A, fig. 11, shows the cell structure of the optic lobes at about 

 the time of the closure of the neural canal. The cells are of 

 the same general forn as those of the optic vesicles but they 

 contain less pigment. This early differentiation of the optic 

 lobes, therefore, is a differentiation of outer form not of cell 

 structure. 



Thus not only may the anlage of the retina be located on the 

 surface of the embryo while the neural plate is still open but 

 also what appears to be its ganglion, the anlage of the later 

 visual center in the mid-brain, may be located just behind the 

 retinal anlage. It seems proable that the ancestors of the verte- 

 brates possessed an eye and optic ganglion lateral to the brain 

 on either side and that these organs were later infolded into the 

 brain when the central nervous system was transformed into a 

 closed canal. This theory offers a reasonable explanation of 

 the fact that the mid-brain unlike the rest of the tube has at the 

 very outset a thickened layer of nerve elements on the roof and 

 sides, the tectum opticum. Indeed such an explanation has 

 before been hinted at by those authors who have endeavored to 



JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3. 



