460 Journal of Comparative Neurology and Psychology. 



The distal part of the cell may contain a few yolk granules, 

 which are usually small and of indistinct contour, as shown in 

 Fig. 7. This portion does not stain readily, shows a distinct 

 membrane, and is free from yolk granules. Such polarity is 

 most marked in the cells (Fig. i, n c.) near the center of the 

 neural plate. From here outwards, towards either side of the 

 neural plate, the nuclei are rounder, the cells shorter, and the 

 granules encroach more and more on the clear portion of the 

 cell. 



As soon as the neural plate is well defined (Fig. 2, n.p.), 

 the outlines of the individual cells become less distinct, though 

 each presents the same general characteristics observed in the 

 preceding stage, as represented in Fig. 7. The neural plate is 

 now composed of two or three layers of columnar cells. The 

 neural cells forming the upper layer show the portion spoken of 

 above as free from yolk granules. They are so arranged that 

 a clear band stretches from one margin of the plate to the oth- 

 er. The ectoderm of the remainder of the embryo becomes 

 two-layered as it approaches the neural plate, and ends abrupt- 

 ly at the margins of the latter, which already shows an incipi- 

 ent infolding (Fig. 2, n.p.). The further development of 

 the neural plate is characterized by an increase in the number 

 of cells composing it, and by an approximation of its margins, 

 which grow together to form a tube, as shown in Figs. 3, 4 

 and 5. 



By the time the anterior part of the neural canal is formed 

 and the Arilagcn of the optic vesicles appear, the neural tube 

 has the general shape of a spherical triangle, the two basal an- 

 gles being formed by the Anlagen of the optic vesicles (Fig. 3, 

 o.v.'). The plane of the section represented is through a part 

 where the canal has not quite closed. The wall is composed 

 of many layers of cells, and varies in thickness in different local- 

 ities. It is, in general, thinner in the region of the future op- 

 tic vesicles. The cells forming the lining layer abut upon the 

 lumen in an irregular line, as represented in Fig. 3. 



The nuclei («.«.) in the wall of the neural canal vary great- 

 ly in outline. Some are round, some oblong, while a large 



