472 Leland Griggs. 



to be true that scattered groups of pigment-bearing cells are 

 found everywhere in the lining layer of the neural canal but the 

 cells of the retinal spot bear more pigment than is found in any 

 other region. Section A, fig. 11, shows the retinal spot (e) 

 drawn to a large scale and section B shows the pigment-bearing 

 cells still more highly magnified. These cells are just beginning 

 to assume a colamnar form such as Eycleshymer has figured for 

 relatively younger embryos of Ran a palustris and other amphib- 

 ians. This pigment is collected at the ends of the cells along 

 the walls. At this stage then the anlage of the retina may be 

 clearly identified in sections as well as in surface views. 



Of perhaps still greater significance is the development of the 

 optic lobes from a region lying lateral to the neural plate. A 

 deep groove, the lateral infolding, has already been described as 

 forming on each side of the procephalic lobes (li, fig. 4, A). As 

 the neural crests fold over, these grooves disappear but at the 

 same time in the same region between the neural crests and the 

 neural plate appear a pair of low ridges or lobes (ol, fig. 8). Each 

 lobe is bounded anteriorly by the optic vesicle and posteriorly 

 by the cerebellar crest. At first rather indistinct in outline it 

 soon becomes very prominent and clear cut (ol, fig. 9). As 

 its growth continues it bulges backward as may be inferred from 

 the pear-shaped form which it assumes (fig. 9, B, D). It may 

 push back slightly under the cerebellar crest (fig. 9, D). It 

 becomes bilobed as can be seen better in the next stage although 

 the beginning is shown in fig. 9, D. 



A study of sections confirms this account of the early appear- 

 ance of the optic lobes. Fig. 10, B, is a section cut longitudi- 

 nally through the fore-brain vesicle and optic lobes in a plane 

 shown by the diagram, A. The optic lobes appear as thickened 

 walls of the neural tube lying directly behind the retinal spots. 

 The walls of this region are slightly thicker than those of the 

 fore-brain vesicle and the diameter of the canal is considerably 

 less, a condition already shown in fig. 9, C, a drawing of a halved 

 embryo. Section C, fig. 10, is cut through the same plane as 

 B but the embryo is a little older, as appears from the fact that 

 the neural canal is now closed. In comparing the two sections 



