iCiinli iiiicil J'rtiiii /"/(/< ''''I 

 luo iiii'diilla aiicrii's [M.A.). Here as clsowlicrc, the cpciKlyina siinouiidiii^ 

 the rourtli ventricle doubt less functions as a inodilieil chorioid plexus, discharg- 

 ing eorebro-spinal fluid into the fourth ventricle. It will be seen that the cavity 

 of the fourth ventricle is smaller than the i)e(!uliarly modified central canal and 

 roof exi)ansion cavity of the Polistotrema spinal cord portra\cd in h^ures 10 to 

 13. X 25. 



65 A more caudal section through the extreme tip of the posterior lobes of 

 the mesencephalon (cerebellum of Miss Worthington), its exact plane being 

 indicated by line 65 in figure 63. It will be seen that the fourth ventricle of the 

 embryo has in this region of the adult become reduced to three small longitudinal 

 canals (.l.i V.), which are nnbedded in a rather large, dense, and vascular epen- 

 dymal mass. The most dorsal of these canals contains Reissner's fiber. Here 

 again the ependymal walls are probably functional as a modified chorioiod plexus. 

 Very shortly these canals reunite and continue some little distance caudad as 

 a small central canal, no larger than the central canal of the spinal cord. X 25. 



66 Transverse section through the posterior end of the medulla of the same 

 series as figure 64. The exact plane of the section is indicated by line 66 in figure 

 63. It passes through what has been designated as the posterior dilation of the 

 fourth ventricle {P4V), which is nothing more than a fair-sized centrally lo- 

 cated cavity, the remains of a much larger embryonic fourth ventricle, surrounded 

 by a great mass of vascular ependyma. The center of this cavity contains a 

 fibrillar feltwork (S.C.F.) composed largely of coagulated cerebro-spinal fluid 

 and some ependymal cilia. Here as more anteriorly we probably have a modi- 

 fied chorioid plexus, the ependymal walls and their blood vessels secreting and 

 filtering eereliro-spinal fluid into the fourth ventricle. X 25. 



ABBREVIATIONS 



AtV., anterior fourth ventricle M.A., medulla artery 



B.V., blood vessel Mes'., posterior lobes of the mesen- 



Ep., ependyma cephalon, cerebellum of Miss Worth- 



Hab. B., habenular body ington 



Inf., infundibulum P4V., posterior fourth ventricle 



M., mesocoele or mesencephalic ventri- S.C.F. , cerebro-spinal fluid 



cle S.M., sinus mesocalicus 



M'., anterior portion of the mesoca?le V.M.R., motor V root 



or sub-commissural canal of Nicholls 4V., fourth ventricle 



M"., dorsal portion of the mesocoile VIII. G., auditors ganglion 



or optocoel and posterior portion of X.R., vagus root 



the optocoel of Nicholls 



PLATE 13 



EXPLANATION OF FIGURES 



67 to 69 Tliree transverse sections through the brain region of a 37 mm. 

 Amphioxus. No blood vessels were seen in any of these sections, but the mem- 

 branous neural canal is surrounded on three sides by enormous veno-lymphatic 

 sinuses, and the structure of the central nervous system is to a considerable extent 

 made up of rather coarse supporting tissue, making infiltration an easy method 

 for nourishing the brain. Drawn with an Kdinger-Leitz drawing apparatus and 

 reduced one-half in reproduction. 



{Con tinned on page 70) 

 6S 



