SPINAL CORD AXD MEDULLA OF CYCLOSTOMES 37 



considerable pressure on the thin and phistic roof plate. The 

 tela chorioidea is differentiated as an organ for the production 

 of cerebro-spinal fluid before blood vessels have entered the 

 central nervous system, at a time when the nutritive function 

 of this fluid is important. 



3) The appearance of a third stage in the roof expansion of 

 the medulla, due to a pontine flexure, is of little significance 

 save in the higher vertebrates, where it was held that without 

 cerebro-spinal fluid confined in the ventricle there would be no 

 reason for maintaining that a further expansion of the roof plate 

 would take place from the action of a pontine flexure: more than 

 likely, the roof plate would have been folded up within the ven- 

 tricle. In His' experiment with the bending upward of a dor- 

 sally-slit piece of rubber tubing, the elasticity of the rubber 

 tubing, which forced apart the cut surfaces, would be com])arable 

 to the action of the cerebro-spinal fluid under moderate pressure 

 within the ventricle, which factor His has ajiparently disregarded. 



4) In the adult Amj^ihioxus there is nothing which for a cer- 

 tainty could be homologized to the fourth ventricle and its ex- 

 panded roof plate. Two isolated cavities in the region of the 

 roof plate, which might be taken for the anlage of the fourth 

 ventricle, ai;)pear to the writer to be nothing more than vestiges 

 of a much larger emliryonic central canal. If Amphioxus pos- 

 sesses no fourth ventricle in the adult we may saf(>ly conjecture 

 that more primitive vertebrates had a central nervous system 

 in which there was no distinction between medulla and spinal 

 cord. 



5) In an attempt to trace the phylogenetic history of the roof 

 expansion of the fourth ventricle in living vertel)rates, the pecu- 

 liar modification of the fourth ventricle in the adult Polistotrema 

 (Bdellostoma) should be recorded here even though it has been 

 accurateh' described b}- Sanders, Holm, Miss Worthington, 

 Sterzi, Cole, and Nicholls. From the adult it is evident that the 

 well-formed fourth ventricle of the embryo has become trans- 

 formed through a process of centralization to a deep-seated canal, 

 for the most part no larger than the central canal of the spinal 

 cord. Of especial interest is the fact that its anterior and pos- 



