20 WILLIAM F. ALLEN 



of the lateral plates. Had a similar increase of nerve fibers 

 occurred in the dorsal plate and in the dorsal portion of the 

 lateral plates, the central canal of the medulla might have been 

 completely obliterated. On the contrary, the dorsal cavity 

 and the roof plate ha^e been slightly expanded, apparently 

 through an increased internal pressure from the embryonic 

 cerebro-spinal fluid. One evidence that increased pressure 

 may exist was found in the marked decrease in the size of the 

 central canal of the cord and of the ventral part of the canal in 

 the medulla region. 



A second evidence is found in the changes that the walls of 

 the fourth ventricle are undergoing preparatory to becoming a 

 functional organ for the production of cerebro-spinal fluid. In 

 connection with the expansion of the roof plate it will be seen 

 that the dorsal mesenchyme has become decidedly vascular 

 (fig. 46, B.V.), making easy an infiltration process into the 

 ventricle. Also ependymal cells, surrounding the ventricle, are 

 taking on form and are probably assuming a secretory function, 

 if they have not acquired one previously. From the recent 

 work of Dandy and Blickfan based on the action of certain drugs 

 and on the chemistry of cerebro-spinal fluid, it is evidence that 

 cerebro-spinal fluid must be a product of secretion as well as of 

 infiltration and diffusion. It is a well-established physiological 

 fact that certain secretory cells, as for example the salivary 

 glands, may assume a definite polarity, and produce a secretion 

 against a very strong pressure, even stronger than that of the 

 blood. Some investigators hold that lumina in glands are the 

 result of pressure from secretion. 



in the light of these facts, it is fair to assume that the marked 

 lateral expansion of the fourth ventricle and its roof plate 

 exhibited in the 26 day series (figs. 50 to 52) are the direct result 

 of internal pressure caused by the marked increase in cerebro- 

 spinal fluid. The general increase in size of the ventricle to- 

 gether with the marked convexity of the roof plate and the 

 concavity of the internal surface of the lateral plates certainly 

 suggest internal pressure from the cerebro-spinal fluid. It 

 should be noted in figures 50 to 52 that this expansion of the 



