LoEB, Optic Vesicles of NectJirus. 463 



In the next stage (Fig. 6, o.v.), the optic vesicles are join- 

 ed to the neural canal by a short hollow stalk. The nuclei 

 lying in the wall of both the neural tube and the optic vesicles 

 (n.n.) are oval or round, there being no ameboid forms present. 

 By this time, the cellular elements have disappeared from the 

 cavities of both the neural canal and the optic vesicles. A 

 minimal amount of detritus can be detected, but this is proba- 

 bly of accidental origin ; at any rate it bears no resemblance to 

 what has been described and depicted in the earlier stages. 

 During the later development of the embryo, the lumen re- 

 mains clear until it is obliterated by the approximation of the 

 outer and inner layers of the optic cup. 



Of special interest are; (a) the nature, (b) the origin, and 

 (c) the fate of the structures found in the neural canal and optic 

 vesicles. While these questions cannot be answered to com- 

 plete satisfaction, enough is known to show that they are prob- 

 ably of great significance 



In regard to their nature, there is hardly any doubt that 

 they are cells. They have the form and physical properties of 

 cells, some possess a nucleus, and many contain yolk granules. 

 They are too numerous to be accidental, and too regular and 

 characteristic to be the product of coagulated albumin. Their 

 number, regularity, and position, coupled with the general 

 appearance of the sections, speak against the assumption that 

 they are portions of long columnar cells cut obliquely. Taking 

 all these facts into consideration, it is probably correct to regard 

 them as cells which in some way have passed into the lumen of 

 the canal. 



As to the origin of the cells, it would seem that they are 

 derived from the cells in the wall of the neural canal, including 

 the optic vesicles, in one of two ways ; (a) by a migration of a 

 part or all of the cell into the lumen of the canal, and (b) by a 

 mitosis of the cells lining the canal, with extrusion into it of 

 one of the daughter cells. Numerous examples of the former 

 process are found, to a few of which reference has been made 

 (Figs. 9, 10, II, 12, 13, 14). Examples of (b) are more rare- 

 ly found, but are undoubtedly present. 



