SPINAL CORD AND MEDULLA OF CYCLOSTOMES 31 



When the roof expansions in the spinal cord of this series of 

 Polistotrema are compared with the fourth ventricle of the higher 

 vertebrate embryos, it is evident that the similarity is onlj- super- 

 ficial, for the later stages in the medulla oblongata (action of 

 pontine flexure on a thin-roofed neural tube full of cerebro-spinal 

 fluid under moderate pressure), are dependent on factors which 

 are not present in the spinal cord of Polistotrema. The extreme 

 caudal end of the roof plate of the fourth ventricle in the higher 

 vertebrates, however, has not been affected by these later fac- 

 tors, and presents a condition where comparison is made po.ssible. 

 From a compari.son of the roof plate expansion in the 23 nmi. 

 human embryo (figs. 26 and 27, R.Ex.), in the 15 nmi. human 

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

 (fig. 31, R.Ex.) with the second roof plate expansion of the 

 spinal cord in the 20 cm. Polistotrema series (figs. 4 and 5. 7?. 

 Ex. 2, and figs. 15 to 17, R.P.Ex. 2) and with the third roof plate 

 expansion of the same .series (figs. 4 and 5, R.Ex. 3), it is evident 

 that the same main factor is present in all, namely, a migration 

 outward and upward of certain roof plate ependymal cells to 

 fonn an enlarged dorsal cavity. In each case the purpose of 

 this structure is to form an organ for the production and storing 

 of cerebro-spinal fluid. As soon as these structures as.sumed 

 the function of infiltrating and secretory organs their walls 

 l)ecame further expanded from internal pressure of the cerebro- 

 spinal fluid. 



From a re\iew of the early stages of the formation of the roof 

 expansion in man, the pig, Amblystoma, Squalus, and Petro- 

 myzon, it is fair to assume that the source and early development 

 of the roof expansion of the medulla are identical to the three 

 similar roof plate expansions, described for the spinal cord of a 

 20 cm. Polistotrema. It is possible that both had their plujlo- 

 genetic anlaqe as mutations, the former from some primitive 

 vertebrate, and the latter from a normal Polistotrema, that both 

 were useful and dominant characters, and in case of the medulla, 

 where the animal was allowed to reproduce, this character became 

 preserved for the race. 



