SPIXAL CORD AND MEDULLA OF CYCLOSTOMES 21 



fourth ventricle has produced a considerable secondary splitting 

 in the concrescence of the lateral plates recorded for the pre- 

 vious series. Apparently this fissure did not penetrate so deeply 

 in the region of the auditory vesicles (fig. 51, .4?^c/.T'.) as it did 

 in front of them (fig. 50) or behind them (fig. 52), which probably 

 indicates that the growth of the auditory vesicles in some way 

 operated against the further splitting of medulla. It is signifi- 

 cant that the expansion of the roof plate and the development 

 of its mechanisms for infiltration and secretion occur at a time 

 prior to the entrance of blood vessels into the wall of the brain 

 and cord. The large production of cerebro-spinal fluid at this 

 time is evidence that it serves some nutritive function. 



In figure 54 we have a median sagittal section through the 

 brain of a 26 day Petromyzon, representing a stage similar to 

 that of figures 50 to 53. Especial attention is called to the 

 fact that figure 54 demonstrates conclusively that the pronounced 

 roof expansion displayed in figures 50 to 52 ha^ occurred ivithoui the 

 aid of a pontine flexure. The slight convexity of the floor of the 

 medulla can be attributed to the increase of fibers. Also earlier 

 and later series revealed that Petromyzon possesses no pontine 

 flexure. Attention should be called to the fact that the marked 

 convexity of the cephalic end of the thin roof plate of the fourth 

 ventricle (C.C.) in figure 54 gives every appearance of being 

 under internal pressure from cerebro-spinal fluid. This section 

 also makes clear, as Sterzi has previously shown, that the fourth 

 \'entricle is formed from the dorsal portion of the embryonic 

 central canal; while the central canal of the cord is formed from 

 the ventral portion. 



The following observations were noted in connection with the 

 appearance of the fourth ventricle in Petromyzon, beginning at 

 a stage when the medulla was a solid cord of undifferentiated 

 nuclei in a syncytium of protoplasm: 1) The ependymal nuclei 

 migrated a short distance to either side of the median sagittal 

 plane leaving a narrow strip of granular protoplasm in the center. 



2) A median sagittal furrow or seam appeared in the central 

 protoplasm, extending from the roof plate to the floor plate. 



3) An isolated cavity appeared at the dorsal and ventral ends 



