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5, 6 and 7, the margins of the neural tube are approximated in the 

 middle plane, and they present the appearance of two parallel ridges 

 lying above the vesicles, and nearer to the middle line. 



Fig. 3, shows a camera tracing of a horizontal section, passing 

 through this system, of the embryo drawn in Fig. 2. It shows, that 

 the optic vesicles (op. v.) are the front ones of a succession of serially 

 arranged pits, and that the rounded eminences on the surface corre- 

 spond to the cuppings within the walls of the neural tube. The various 

 sections show the morphology of the system in a satisfactory manner. 

 One might expect to find differentiations of the cells in these pits, but, 

 this early period is not one of marked histological differentiation, even 

 the true optic vesicles do not show distinctive cellular differentiations 

 till a later period. There is no marked histological differentiation in 

 the cupped regions, but, germinating cells are apparently more frequent 

 within the areas of the vesicles than within the adjacent brain walls. 



This is the period of greatest development of the system as a 

 whole. The specimen drawn I estimate to be between the 24 and 25 

 hour-stage of development. It has six fully formed mesoblastic somites 

 on one side (the right), and five fully formed ones, and a sixth not 

 fully differentiated, on the other side. In front, the neural groove is 

 open in the form of a wide V, and it is closed to a point nearly as 

 far back as the beginning of the mesoblastic somites. 



In order to remove as far as possible the personal equation, I 

 have had sketches made of the embryo under consideration, by three 

 different persons, and have had the vesicles counted, independently, 

 by several others. The observers all agree as to their number and 

 general characteristics. The embryo was also successfully photographed, 

 showing the series of accessory vesicles at a magnifying power of 

 about thirty diameters. Finally, it was cut into horizontal sections 

 and reconstructed according to the method of Born. The reconstructed 

 model bears out the interpretation based on the other lines of obser- 

 vation. 



A careful study of Figs. 1 and 2, as representing closely related 

 stages, will show that the succession of vesicles represented in Fig. 2, 

 is a part of the original optic differentiation of Fig. 1. The front 

 protuberant part of the optic ridge has been on each side, converted 

 into the optic vesicle, but, only a portion of it has been so used, for, 

 the optic vesicle in Fig. 2, is shorter than the corresponding bulge 

 in Fig. 1. The remainder of the latter has been transformed into the 

 succeeding vesicles. The six, smaller vesicles exhibit such a close 

 anatomical continuity with the primary optic vesicles, that they may 



