iCoiilnnnil frniii piiii, US) 



I'lif iii.ot ;iiilirii)i- si'ctidii, li-iuri' (17, piisscs tliroiijili t lie iinU^rior vcjil.ricdi; at 

 its lii^ilu'sl i)()iiit, wliicli is a sliort (lislaiicc hcliiiid tli(^ nciiroporc. This ventri- 

 cle has no (h)isal dilation suggestive of the fourth ventricle. What dilation occurs, 

 is niedian and ventral. Cilia-like processes from the border of the cells enter 

 the cavity. I f (lie cpciidyiual cells are not secretory it is possible that the cere- 

 bro-spinal lliiid ul' Atnphioxu.s does not differ from the serum of the adjacent 

 veno-lymphatic siiuiscs. If nerve cells occur in this region they are small, and 

 in ordinary i)reparat ions indistinguishable from ependymal cells. X 125. 



OS (50 microns behind figure 67. The large central canal of the eml)ryonic 

 brain has evidently become reduced in this region to a ventral central canal 

 {C.C.) and a small dorsal isolated cavity {V 2.). This isolated dorsal cavity can 

 not be comi)ared with the fourth ventricle of higher vertebrates. It is rather 

 to be looked upon as a vestigeal structure, which may aid in the infiltration of 

 lymph from the outer veno-lymphatic sinuses. X 125. 



69 Taken from a section 530 microns behind figure 68. It passes through 

 that part of the brain in which there are accumulated a great number of giant 

 cells {M' .C .) in the region of the roof plate. As in figure 68, there is an isolated 

 cavity near the dorsal surface, which was probably a portion of the large embry- 

 ofiic central canal, but which in the adult is separated from the central canql and 

 from the more anterior isolated dorsal cavity by ependyma. It seems best to 

 the writer to regard this and the preceding dorsal cavity as vestigeal structures. 

 X 125. 



70 (See next plate.) Transverse section through the anterior spinal cord 

 from the same series as the three previous figures. Observe that the Amphioxus 

 spinal cord is not depressed as is the Cyclostome spinal cord, but is indented ven- 

 trally by the notochord. The central canal, which in some places exists as a 

 dorso-ventral cleft, is almost obliterated here by the ingrowth of ependymal 

 tissue. X 125. 



71 Cephalic transverse section through a portion of the spinal cord, menin- 

 geal membranes, neural arch, notochord, spinal ganglion, and sensory root of 

 an adult Polistotrema. Observe the depression of the spinal cord, its ventral 

 indentation, the ventral or permanent central canal {C .C .), which contains Reiss- 

 ner's fiber, and immediately above, the dorsal portion of the embryonic central 

 canal, which is here more or less filled with ependymal cells and their processes. 

 It will be seen that the gray matter is as much flattened out as is the cord it- 

 self, and the ventral horn and motor cells are crowded laterad, while the dorsal 

 horn, substantia gelatinosa {S.G.), is apparently median and dorsal. Within 

 the neural arch there is abundant room for a spherical spinal cord. The cord is 

 held in place by the usual meningeal membranes. X 70. 



ABBREVIATIONS 



Ar., Arachnoidea Nc, notochord 



C.C, central canal P.M., pia mater or meningeal mem- 



C.T., white fibrous connective tissue bi-ane of the younger stages 



D.M., dura mater S.G., substantia gelatinosa 



D.S., dorsal veno-lymphatic sinus Sp.G., spinal ganglion 



Ep., ependyma S.R., sensory or dorsal spinal nerve 



L.S., lateral veno-lymphatic sinus or root 



anlage of the same Suba.S., subarachnoid cavities 



M'.C, Miillerian or giant cells Subd.S., subdural spaces 



M.F., Miillerian or giant fiber W.M., white matter 



Myo., myotomes V.L, anterior ventricle Amphioxus 



A^.^., membranous neural arch V.2., vestiges of the embryonic central 

 N.C., nerve cell canal in Amphioxus 



70 



