SPINAL CORD AND MEDULLA OF CYCLOSTOMES 2i 



the dorsal expansion of the roof plate exhibited in figures 79 and 

 80. Apparently this expansion of the roof plate in the pig has 

 been gradual, for absolutely no stretching of the roof plate has 

 occurred, except in one place, namely in its central anterior 

 portion (fig. 79). 



A study of these sections has disclosed a direct relationshiji 

 between the expansion of the roof plate and the amount of visi- 

 ble coagulum in the ventricle. Since coagulum does not appear 

 in sections of the early fourth ventricle, but does appear after 

 the tela chorioidea has attained the function of producing cerebro- 

 spinal fluid (as is indicated by its vascularity and the granular 

 appearance of the cells) it is fair to assume that the non-coagu- 

 lable cerebro-spinal fluid found in sections of the early embryos 

 is an embryonic cerebro-spinal fluid, which differs in no way 

 from the ordinary intercellular fluid of other tissues. On the 

 other hand, the coagulum seen in sections after the roof plate 

 has reached the stage of a functional chorioid plexus is evidence 

 of a chemical change in the fluid, which, if a product of secre- 

 tion, is capable of exerting considerable internal pressure and 

 consequent expansion of the roof plate. 



It is apparent that the greater expansion of the roof plate in 

 the pig is produced by the same factors as were recorded for 

 Petromyzon, namely, an early migration outward of the roof 

 plate cells followed by an expansion from within due to the for- 

 mation of cereliro-spinal fluid, plus the action of a conspicuous 

 pontine flexure on a fourth ventricle filled with cerebro-spinal 

 fluid alread}^ under moderate pressure. 



F. Human embryos. For this studj' an 8 and a 15 nun. trans- 

 verse series and a 23 mm. frontal series were available. These 

 embryos were too far advanced to show the earliest stages of 

 the roof expansion of the fourth ventricle. If, however, the 

 extreme posterior end of the roof plate of the fourth ventricle 

 is examined in the 23 mm. embryo (figs. 26 and 27, R.Ex.), in 

 the 15 mm. embryo (figs. 28 and 29, R.Ex.), and in the 8 mm. 

 embryo (fig. 31, R.Ex.), which represents a region of the medulla 

 little affected by the pontine flexure, it is apparent that the roof 

 expansion was caused by identically the same factors as was 



