LoEB, Optic Vesicles of Nectunis. 461 



number are irregular, showing a prolongation in the direction 

 of the lumen. Some of the latter are shown under higher mag- 

 nification, in Figs. 9, 10, II. These represent three cells from 

 the same field, and illustrate different stages of the same phe- 

 nomenon. The body of each nucleus shows a number of scat- 

 tered karyosomes, while the prolongation is more or less striat- 

 ed, the striae running parallel to its long axis. The appearance 

 of the cytoplasm and the nuclei indicates that the cells are 

 migratory. 



Some of the nuclei are undergoing karyokinesis and these, 

 as well as the ameboid forms of nuclei, are more numerous 

 where the walls are thinnest, being especially so in the region 

 of the optic vesicles. In places the cells are seen dividing, 

 where one-half is being thrust out into the lumen. 



The lumen of the canal contains a mass which can be re- 

 solved into three constituents; (i) and most numerous, are round 

 structures slightly larger than the neighboring nuclei, which are 

 massed together and more or less completely fill the lumen. 

 These will be shown to be cells in various stages of degenera- 

 tion ; (2) nuclei and larger remnants of the same ; (3) detritus. 



The first of the contents of the lumen consists of well de- 

 fined cell outlines. They are round, of nearly constant size, 

 having an average diameter of 40//, and are usually clear and 

 devoid of yolk granules, as depicted in Fig. 3, l.c, They are 

 more numerous near the head end of the embryo, especially in 

 the region of the future optic vesicles. 



The nuclei lying in the lumen are, in a varying degree, small- 

 er and less deeply stained than those in the wall of the tube. 

 They may be round or may assume ameboid forms (Figs. 12, 

 13, 14). Some are pear shaped (Figs. 13, 14), with the larger 

 mass extending into the lumen. Many appear (Fig. 13) as if 

 the chromatin is passing out of the basal portion of the cell and 

 becoming rounded up in the distal portion. 



The round forms of the nuclei either show a cell membrane 

 around them, or where this is not distinct, there are yolk gran- 

 ules present, indicating the position of the cell. A few nuclei- 

 lie in the lumen independent of any connections with a cell, but 



