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and become thinner, anrl, when viewed from above, the vesicles seem 

 bordered by a thickened edge of the neural tube. It looks as if the 

 thickened margins of the tube, which are in close contact in Figs. 2, 

 4, 5 and 6, had been pushed apart by the expansion of the dorsal 

 brain wall. While this is not true, the appearance presented serves 

 to make a contrast between the two sets of vesicles. The optic ves- 

 icles, are not widenings of the tube as a whole, but bulgings in the 

 lateral walls only. 



Fig. 9, shows a stage with 12 or 13 mesoblastic somites in about 

 the 31 -hour stage of development. It represents a succession of brain 

 vesicles and, although a well-known stage, is not so frequently seen 

 in figures as stages a little younger and those a little older. 



A direct comparison of Figs. 2 and 9 will be instructive. They 

 both show a succession of vesicles involving the brain walls, and they 

 present enough features of resemblance to show how easily confusion 

 might arise in observing them. The two sets of vesicles are entirely 

 different structures, nevertheless, if we had only the stages represented 

 in Figs. 2 and 9, without any connecting forms, one could not escape 

 the impression that those shown in the former figure, are an earlier 

 stages of those in the latter. But, an examination of the intermediate 

 stages is very conclusive. It shows, that what I have called the "ac- 

 cessory optic vesicles", decline instead of developing progressively and 

 merging into the brain vesicles. They disappear completely before 

 the brain vesicles arise and can have no genetic connection with them. 

 Duval, shows in some of his figures made by transmitted light, an en- 

 largement behind the optic vesicles in the 26-hour stage, which he 

 designates the beginning of the second brain vesicle, but he does not 

 represent a succession of vesicles until about the 29th hour. If it 

 were suspected that the earliest brain vesicle shown by Duval, has 

 possibly some connection with the accessory vesicles, it needs only to 

 be remembered, that the former structures have partly disappeared by 

 the 26-hour stage, and that Figs. 2 to 6 in this paper, show a series 

 of gradations that carry them completely through the stages of dis- 

 appearance. 



Another question suggests itself: Are the accessory vesicles de- 

 scribed in this paper, structures that have been hitherto observed and 

 confused with the cerebral vesicles, or they entirely new structures? 

 A careful examination of the figures that have been published, and a 

 search through the literature, leads me to believe that they are new 

 structures not before mentioned in any descriptions of chick embryos. 

 If this be true, the communicatioa now presented, is not the straigh- 



