115 



when viewed from within the tube, and form bulgings on the outside. 

 The more protuberant anterior part, is the beginning of the optic 

 vesicle, and the hinder, more slender portion, is a continuation of the 

 same differentiation extending backwards, and terminating at a point 

 about two thirds the distance from the front end of the embryo, to 

 the first mesoblastic somite. The whole structure may be called the 

 "optic groove" from within, and the "optic ridge" from without the 

 tube. The term optic groove has been used by Heape, for a cor- 

 responding early condition of the optic vesicle in the mole. In some 

 specimens the hinder portion is, from the time of its first appearance, 

 imperfectly divided into shallow vesicles, but in the individual figured 

 it is merely wavy. 



This stage is followed very quickly by that represented in Fig. 2, 

 in which, the primary optic vesicles {op. v.), have become clearly marked 

 off from the rest of the system. In the specimen just described, the 

 posterior limits of the optic vesicles were uncertain, but now they may 

 be clearly determined. But the optic vesicles are not entirely separated 

 from the rest of the system; just behind them, is a succession of six, 

 similar, smaller, rounded patches of epithelium (ac. v. 1 to ac. v. 6) 

 into which the hinder part of the optic ridge has been converted. 

 These additional vesicles are not exactly uniform in size, the first three 

 are best developed and more rounded in appearance, the fourth is more 

 elongated, and the fifth and sixth less evident and subject to greater 

 variations. 



There are segmental divisions in the chick embryo, during the 

 stages under consideration, that have not yet been fully described »). 

 The series of accessory vesicles shown in Fig. 2, ac. v. 1 to ac. v. 6, 

 correspond pretty closely to those segmental divisions. The optic ve- 

 sicle covers the space of three of them, and the remaining six vesicles, 

 reach backwards to the end of the ninth segmental division. There 

 is not, however, in this regard, ridgid uniformity; in some specimens, 

 the six accessory vesicles cover the space only of five and one half 

 "somites", and the vesicles are not precisely opposite the corresponding 

 somites. But the variation is very slight. 



It should be observed, that this set of vesicles occupy the lateral 

 walls of the neural tube, and do not involve the upper margins of the 

 neural folds as the brain vesicles do (Figs. 8 and 9). In Figs. 2, 4, 



1) Eegarding these segmental divisions in the chick embryo, it is 

 expected that a further communication vs^ill soon be made from this 

 laboratory. 



