SPINAL CORD AND MEDULLA OF CYCLOSTOMES 17 



this section. This furrow or seam represents the position of the 

 future central canal. The protoplasm adjacent to this seam is 

 sufficiently granular to suggest a secretory function at this early 

 stage. 



Figures 33 to 35 also taken from a 10 daj' Petromyzon series 

 exhibit a slightly later stage in the formation of the central 

 canal. Conspicuous cavities (C.C.) have appeared in the dorsal 

 and ventral portions of this furrow, which are not only visible 

 throughout the entire medulla, but are continuous throughout 

 the spinal cord. It should be recorded that a few sections through 

 the medulla possessed slightly larger dorsal and ventral cavities 

 than were represented by these figures, but in no case had they 

 approached each other close enough to unite. In other respects, 

 excepting possibly for a few more fibers in the marginal layer, 

 the structure of the medulla has remained about the same as 

 in the previous series. Emphasis should be made of the fact 

 that each of these cavities, when examined with a higher magni- 

 fication, reveals a certain amount of fine granules, which in some 

 cases may have completely filled the cavity, while in others 

 they are confined to the outer edge, leaving a clear space in the 

 center. The presence of these granules here suggests two proc- 

 esses: 1) disintegration of the central protoplasm, and 2) prod- 

 ucts of secretion. The lateral migration of the nuclei could i)e 

 utilized to support either inference. That such a migration 

 of nuclei would be favorable for disintegration is self evident, 

 and secretory cells are usually characterized by having their 

 nuclei somewhat remote from their lumina. 



In the 11 day series (figs. 3(3 to 38) we find that the two iso- 

 lated dorsal and ventral cavities of an earlier stage have not 

 only united and formed a cleft-like cavitj^ which may now be 

 designated as the typical embryonic central canal, but that the 

 original dorsal and ventral cavities of this canal, especially 

 the dorsal, have increased notably in size. The narrow central 

 portion (figs. 37 and 38) indicates that this is the place where the 

 protoplasm was last to separate. It should not be confused 

 with a similar condition that occurs later, when the central and 

 ventral portions of the lateral plates migrate inward, fuse, and 



THK JOUKNAL OK COMHAHATIVE NM;l:ROL,OOY, VOL. 2b, NO. 1 



