2(i AVILLIAM V. ALLEN 



about this pronounced expansion of the roof plate. Since the 

 closure of the dorsal portion of the embryonic central canal of 

 the spinal cord occurs much later in embryonic life, no increase 

 in pressure from cerebro-spinal fluid could take place from that 

 source. Also no increase was shown in the number of blood 

 vessels outside the roof plate, and no blood vessels had entered 

 the medulla. Consequently the only means of an increase of 

 cerebro-spinal fluid would be through secretion and a slight 

 infiltration from the blood vessels. This section demonstrates 

 yery strikingly, even more so than figure 75, that the roof ex- 

 pansion begins very early in the development of the pig's medulla, 

 as is evident from the fact that nerve fibers have only begun 

 to appear in the marginal layer. 



Between the 6 mm. and 7 mm. stages there occurs a marked 

 increase in the blood vessels in the mesenchyme above the roof 

 plate and a few blood vessels are entering the substance of the 

 medulla. There can be no question that the roof plate is now 

 an efficient organ for the production of cerebro-spinal fluid, and 

 there is a noteworthy increase in the amount of coagulum in the 

 cavity (figs. 75, 76, and 77). The first embryonic cerebro-spinal 

 fluid, probably a mere intercellular fluid, showed little or no 

 coagulum, from the method of fixation and staining used. This 

 may indicate a relative increase of the elements formed by secre- 

 tion. Figure 77 shows that a marked expansion of the roof 

 plate has occurred in the 7 mm. pig. Since the 7.5 and 8 mm. 

 pig embryos have very small pontine flexures, it is evident that 

 this pig embryo has developed a well-expanded roof plate and 

 chorioid plexus without the influence of a pontine flexure. 



The roof plate in the 10 mm. pig (fig. 78, R.Ex.) and in the 

 14 mm. pig (figs. 79 and 80, R.Ex.) has undergone a decided 

 expansion, especially in a dorsal direction. This final expan- 

 sion of the roof plate is unquestionably due to the action of the 

 pontine flexure upon a fourth ventricle filled with cerebro-spinal 

 fluid already under moderate pressure. It would have been 

 impossible for a pontine flexure acting alone on an empty fourth 

 ventricle, as would be implied from His' experiments with bend- 

 ing an empty rubber tube slit dorsally, to have brought about 



