Johnston, Forebrain Vesicle in Vertebrates. 



473 



dibulum" it is completely obliterated. In some embryos it is impos- 

 sible to distinguisli any preoral entoderm beneath the brain in front 

 of the ''infnndibuliim," but in others there is no doubt whatever 

 that a slender rod of cells lying in this position is the remnant of 

 the anterior part of the preoral entoderm. This rod of cells is sur- 

 rounded by cells (mesectoderm) derived from the neural crest above 

 described, which constitute by far the largest part of the cells lying 

 in this position. This mesectoderm forms a considerable mass of 

 cells filling a lens-shaped space between brain and ectoderm and 

 between "infundibulum" and neuropore and also extends as a sheet 



Fig. 9. Neal's Fig. 11 modifled by addition of the terminal part of the 

 neural crest. The figure represents the lateral view of a cleared embryo of 

 Squalus of 24-25 somites and shows the extent of the mesectoderm derived 

 from the neural crest. 



along the sides of the "infundibulum" beneath the optic vesicle. 

 Fig. 9 is a copy of Neal's Fig. 11, PI. 3, of this stage, with the addi- 

 tion of this mesectoderm. 



In one embryo of 26 somites (Fig. 10) I have found the slender 

 rod of preoral entoderm persisting beneath the "infundibulum," but 

 from this time on it is impossible to recognize entoderm in this 

 position. The "infundibulum" has become depressed until it is in 

 contact with the ectoderm and at the same time, whether because of 

 pressure from the "infundibulum" or not, the median part of the 

 preoral entoderm becomes obliterated while its lateral portions form 



