SPINAL CORD AND MEDULLA OF CYCLOSTOMES 19 



established that the central protoplasm in disintegration con- 

 tributed nothing to it. The spinal cord was found to be in about 

 the same condition as the 11 da}^ series. 



Moreover an 18 day series (figs. 42 to 44), some 6 daj's older 

 than the last stage compared, shows pVacticallj' no change in 

 the shape and size of the central canal or expansion of the roof 

 plate, despite the fact that a notable increase of fibers had oc- 

 curred in the ventral and median portions of the lateral plates. 

 It should also be recorded that the intermediate stages, as rep- 

 i-esented by the 14 and 16 day series, exhibited a like state of 

 the central canal. A further interesting condition is revealed 

 from a section of the spinal cord of this series (fig. 45). Here the 

 lateral plates have expanded to such an extent from the for- 

 mation of fibers in the marginal layer, that the inner surfaces of 

 their central areas have nearly formed a complete concrescence 

 at the center of the original central canal. In other words, the 

 inner surfaces of the lateral plates have met, and are about to 

 fuse, leaving dorsal and ventral cavities (CO. The ventral 

 one of these cavities will persist as a pemianent central canal. 

 In Petromyzon this partial closure of the original embryonic 

 central canal of the spinal cord may have considerable bearing 

 on the increase of internal pressure of cerebro-spinal fluid on the 

 walls of the medulla, during this and later stages. We may 

 regard the period from aiiout the 12th to about the ISth day as a 

 period of rest in the formation of the fourth ventricle in Petromy- 

 zon. During this time the processes which would tend to expand 

 the central canal have been met with equal counter forces, which 

 would make for closing it up. 



Transverse sections through the medulla of a 20 day Petromy- 

 zon series (figs. 46 to 48) demonstrate a marked change in the 

 medulla and its central cavity, which cavity has now assumed 

 the form of a fourth \entricle. A marked increase has taken 

 place in the number of fibers in the ventral plate and in the 

 ventral portion of the lateral plates. This together with pres- 

 sure from the growing auditory vesicles (Aud.V.) and the noto- 

 chord, has produced a nearly complete concrescence of the 

 corresponding middle and \entral portions of the inner surfaces 



