'11 WILLIAM F. ALLEN 



of this furrow. 4) The two cavities became connected, forming 

 a cleft-like canal, designated as the typical embryonic central 

 canal. This canal seems to be formed by a disintegration of the 

 central ends of the ependymal cells, now in the form of a syn- 

 cytium. 5) A considerable increase in the size of the dorsal 

 portion of the central canal occurred through the upward and 

 outward migration of some of the roof plate nuclei and a disinte- 

 gration of the inner protoplasm. No similar expansion of the 

 ventral portion of the central canal of the spinal cord took place 

 because its marginal layer became reinforced very early by the 

 addition of nerve fibers. 6) Following the formation of the 

 central canal of the medulla there was an increase of cells and 

 fibers in the lateral walls, but for the space of about six days 

 there was little change in the size of the central canal. 7) Next 

 a very pronounced increase of fibers occurred in the median 

 and ventral portions of the lateral plates, which brought about 

 a complete concrescence of the corresponding inner margins of 

 the lateral plates. 8) A sufficient amount of cerebro-spinal fluid 

 was formed by infiltration and secretion to produce a marked 

 expansion of the fourth ventricle. This expansion pushed apart 

 the dorsal portion of the lateral plates, which had not been 

 thickened by an addition of fibers, and stretched the roof plate 

 to a much greater width. 9) Along with this expansion a 

 secondary splitting of the concrescence noted in (7) took place. 

 This fissure did not penetrate so deeply in the region of the 

 auditory vesicles on account of the mechanical obstacle offered 

 by these vesicles. 10) This expanded roof plate apparently 

 assumed the function of producing cerebro-spinal fluid at a time 

 previous to the entrance of blood vessels into the central nervous 

 system, when its nutritive function would be of importance. 



From a review of the main points in the development of the 

 fourth ventricle in Petromyzon we are warranted in concluding : 

 1) That a well-developed fourth ventricle and tela chorioidea 

 were formed in one of the lowest living vertebrates, Petromyzon, 

 without the aid of a pontine flexure. 2) The best suggestion 

 that has been given for the appearance of such an organ in the 

 medulla rather than elsewhere in the central nervous system is 



