8() WILLIAM V. ALLEN 



st)ur('(\^. First in the lower \ei-tei)rjites, as a conseciuence of con- 

 siderable embryonic cerel:)r()-s|)inal fluid being forced into the 

 fourth ventricle from the closure of the tlorsal and central por- 

 tions of the embryonic central canal in the spinal cord and the 

 ventral portion of the embryonic central canal in the medulla, 

 through the union and fusion of the corresponding portions of 

 the lateral plates. Second, in all vertebrates, through the pro- 

 duction of cerebro-spinal fluid by the walls of the fourth ventricle 

 assuming the role of infiltration, diffusion and secretion. That 

 the ventral portion of the embryonic central canal of the spinal 

 cord was not expanded by the same cause is explained by the 

 fact that it was reinforced at a very early stage by nerve fibers 

 and supported by a growing notochord. c) A third and final 

 stage in the expansion of the roof plate in the higher vertebrates 

 was brought about by the appearance of a pontine flexure acting 

 on a thin-roofed medulla filled with cerebro-spinal fluid, itself 

 under moderate pressure. 



2) It is the opinion of the writer that the most important 

 factors in bringing about the expansion of the roof plate of the 

 medulla are those concerned in the second stage described above. 

 For a considerable period of time these factors apparently work 

 in conjunction with the forces of the first stage in resisting counter 

 ingrowths of the lateral plates which would tend to close up the 

 central canal. Since it was shown that no pontine flexure oc- 

 cured in Petromyzon, it can be concluded that the rather exten- 

 sive expansion of the roof plate in the medulla of this genus was 

 accomplished solely through the factors entering into the first 

 and second stages. For the second stage it was recorded in 

 most cases, especially well shown in the pig, that the size of the 

 fourth ventricle and the expansion of its roof plate bear a close 

 relationship to the amount of coagulum seen in sections. Since 

 this coagulum did not appear in sections until after the roof 

 plate had attained the function of producing cerebro-spinal fluid, 

 it probably indicates a chemical change in the cerebro-spinal 

 fluid, the products of secretion now being added to the tissue 

 fluids which entered the ventricle b\" infiltration. As a product 

 of secretion, the cerebro-spinal fluid would be capable of exerting 



