462 Journal of Comparative Neurology and Psychology. 



from their small size, faint staining capacity, and general indis- 

 tinctness, we may conclude that they are degenerating. 



The third constituent of the contents of the lumen is a 

 debris composed of fragments of yolk granules, disintegrating 

 nuclei, and remains of cell membranes. 



In the next stage (Fig. 4) the development of the embryo 

 has advanced so far that the optic vesicles {o.v.) are distinct 

 evaginations of the neural canal. The figure shows a section 

 through the neural canal and the centers of the optic vesicles. 

 The nuclei of the neural cells lying next to the lumen show a 

 tendency to assume ameboid forms which however, are not as 

 pronounced as in the preceding stage. They are most numer- 

 ous in the dorsal wall of the canal, and in the superior wall of 

 the optic vesicles. Along the margin, nuclei undergoing degen- 

 eration are occasionally seen. 



The center of the lumen is entirely free from cellular ele- 

 ments or their remnants. The remainder of the neural canal 

 shows the cell outlines described above, some nuclei and a few 

 yolk granules. The cells of the optic vesicles are more compact, 

 larger and more irregular than those lying towards the center 

 of the canal. Where the section does not pass exactly through 

 the center of the optic vesicles, but on either side, the central 

 space is larg,er, and the mass of cell outlines lies in or near the 

 optic vesicles. The nuclei lying in the lumen are round and no 

 longer show ameboid forms. In Fig. 4, there are shown nuclei 

 {l.n.) with well defined chromatin threads, while in other nuclei 

 these threads have fused into a dense mass. 



Fig. 5 represents a section through the neural canal and 

 optic vesicles {o.v.) of the next stage. The cells in the wall 

 are arranged in two or three layers. The dorsal wall of the 

 canal is still from three to six layers of cells in thickness, but 

 most of the nuclei are ameboid in shape or undergoing mitosis. 

 In the lumen the conditions described above are accentuated. 

 The clear central space is very large and extends nearly from 

 the dorsal to the ventral wall of the canal. The remainder of 

 the lumen, especially the optic vesicles, is filled with cells and 

 nuclei in various stages of degeneration. 



