SPIXAL CORD AXD MEDULLA OF CYCLOSTOMES 13 



from its position immediately above a dorsal group of large nerve 

 cells (M'.C). From the figures by Hatschek, Willey, Sterzi. 

 and others it is evident that these isolated roof cavities described 

 above, were once a part of a common central cavity of the 

 embryonic brain, which later in development, became isolated 

 'through an invasion of ependjaiia, and it is entirely possible 

 that this region of the central cavity in the embryo was much 

 more suggestive of the fourth ventricle. Judging from the adult 

 alone they may he looked upon merely as vestiges of the embryonic 

 central canal. 



B. Folistotrema i Bdellostoma) 'Sly embryonic material <>f 

 Polistotrema confirms the statements of Price, von KuptTcr. 

 and Dean that Polistotrema possesses well-developed ventricles 

 in the embryo; the expansion being fully as great as in a similar 

 stage of Petromyzon. As development proceeds the lateral 

 plates increase in thickness from additions of fibers and cells 

 until the fourth ventricle becomes reduced to a canal, but little 

 larger than the central canal of the spinal cord. 



Sanders, Holm, Miss Worthington, Sterzi, Cole, and Xicholl> 

 describe and figure the fourth ventricle about as it is shown in 

 figure 63. There is some little discrepancy in the terminolog>- 

 used, due largely to the differences (if opinion as to whether oi- 

 not Polistotrema has a cerebellum. If the jiosterior lobes of the 

 mesencephalon should in the light of future investigation turn 

 out to be a cerebellum, then the boundaries of the metencephalon 

 will have to be carried further forward than we have indicated. 

 and the so-called sinus mesoca?licus of XichoUs iS.M.) will 

 have to be called the anterior dilation of the fourth \entricle of 

 Miss Worthington's description. 



My transverse series through the brain of Polistotrema show 

 the condition of the ventricles to be almost identically the same 

 as Sterzi and Nicholls found them. Until the embryological 

 and functional areas of the brain have been better worked out 

 it seems advisable to the writer to let the posterior border of 

 Nicholls' sinus mesocoelicus (fig. 63, S.M.) mark the boundry 

 line between the mesencephalon and the metencephalon, and to 

 regard his isthmic and ventricular canals (figs. 63 and (io. A. 4V.) 



