SPINAL CORD AND MEDULLA OF CYCLOSTOMES 25 



tions of the lateral plates in Squalus (fig. 72, C.C.C.) similar 

 to the condition in Petromyzon. In Neeturus (fig. 73) and in 

 Amblystoma the roof plates are expanded to such an extent that 

 no fusion of the lateral plates takes place; nevertheless, the 

 thickening of the walls tends to bring about a reduction in the 

 size of the fourth ventricle and consequent pressure on the 

 cerebro-spinal fluid. The probability of a decided increase of 

 internal pressure from the cerebro-spinal fluid resulting from the 

 reduction in caliber of the embrj'onic central canal of the spinal 

 cord by at least two-thirds, is more e\'ident, if attention is directed 

 to the fact that the length of the central canal of the spinal cord 

 is fully twenty times that of the fourth ventricle. In both 

 forms the above changes took place before the blood vessels had 

 reached the dorsal surface of the roof plate or entered the 

 medulla. A median sagittal section through a Squalus embryo 

 of the same stage as figure 72 shows that no pontine flexure has 

 appeared. 



E. Pig embryos. For making observations on the development 

 of the tela chorioidea of the fourth ventricle in manunals the 

 writer had access to a very complete set of frontal sections of 

 pig embryos from a stage of 4 or 5 mm. up to 14 mm. embryos. 



The earliest section (fig. 75), which is from a 4 or 5 mm. pig. 

 discloses that the fourth ventricle has only begun to expand. 

 A large part of this expansion could be attributed to an outward 

 and upward migration of the roof plate cells, and a part to in- 

 ternal pressure from cerebro-spinal fluid, of which traces are 

 beginning to appear as a coagulum (S.C.F.). It should be noted 

 that the first blood vessels are appearing above the basal por- 

 tions of the roof plate, while none have at this stage entered the 

 medulla. Also the ependymal cytoplasm is sufficiently granular 

 to suggest a secretory process, and finally the roof plate has 

 begun to expand before any nerve fibers have appeared in the 

 marginal layer. 



A transverse section of the medulla of a (3 mm. pig (fig. 7(3) 

 revealed a considerable expansion of the roof plate without the 

 aid of a pontine flexure. It is apparent that an increase in the 

 amoimt of cerebro-spinal fluid is the main factor in bringing 



