(( mill ini( (I. J idiu jxii/i 72} 



7;") to SO rcprcscMl livt- t r.'iiisvcrsc sections throiitih I he il(vcloi)iiif; lourtli 

 vcntricli" .•mil roof phitr cxpaiisioii iti |)iti cinlnvos from '> nirii. up (o 14 mm. 

 With till" ("\ccpt ion of lijiuic 7."), w hicli is I'loin my collccf ion, the rcunaininfj; figures 

 are from frontal series helonging to the Institute of Anatomy. That there is 

 a dirert rolationsliip between the amount of visible coagulum in the form of a 

 fibrillar feltwork and the expansion of the roof plate is evidenced by the fact 

 that this ct)agulum does not ajipear in the early embryos l)efore the roof plate 

 has assumed the ai)pearance of an organ capable of the production of cerebro- 

 spinal fluid (as indicated by vascular supply and granular appearance of the 

 cells). It may l)e inferred that the earliest non-coagulablc cerebro-spinal found 

 in the earliest stages is an cmbi yonir fluid which differs in no way from the ordi- 

 nary intercellular juices, but that the appearance of coagulum at the time when 

 the roof plate has attained the appearance of a functional chorioid plexus is 

 indicative of a chemical change in the fluid, which if a product of secretion is 

 capable of producing a marked increase of internal pressure in the corebro-spinal 

 fluid and consequent expansion of the roof plate. 



75 From a transverse section through the medulla of a 5 mm. (or less) pig 

 embryo, in the region of the auditory vesicle. It will be seen that the peripheral 

 branches of the intersegmental blood vessels have about reached the roof plate, 

 but no blood vessels have entered the medulla. The protoplasm of the inner 

 margin of the ependymal cells is sufficiently granular to suggest a secretory 

 function. The small amount of coagulum in ventricle is probably the result 

 of secretion, but the cerebro-spinal fluid has probably not exerted much inter- 

 nal pressure. X 70. 



76 Transverse section of a 6 mm. pig medulla through the widest portion 

 of the fourth ventricle, namely, at the level of the V ganglion. This is the only 

 portion of the roof plate to have undergone any stretching of its cells, and this 

 is confined solely to the most centrally located cells. This section shows a con- 

 siderable increase in the size of the fourth ventricle and expansion of the roof 

 plate, together with some increase in the amount of coagulable cerebro-spinal 

 fluid (S.C.F.) and an increase in the number of blood vessels above the roof plate; 

 but no blood vessels have entered the substance of the medulla. At this stage 

 the pontine flexure could not have been a factor in producing the roof expansion. 

 The collapsed appearance of the roof plate at its center is not natural, but rather 

 a result of the preparation of the material. X 39. 



PLATE 15 



EXPLANATION OF FIGURES 



77 Taken from a transverse section through the V ganglion of a 7 mm. pig 

 embryo. Note the increase in the number of blood vessels above the roof plate, 

 which together with the increase in the coagulable cerebro-spinal fluid suggests 

 a functional chorioid plexus. The pontine flexure in this stage is too slight to 

 have any effect on the expansion of the roof plate. It should also be recorded 

 that a few blood vessels have entered the outer surface of the medulla. As in 

 the previous series the sections have suffered a collapse of the roof plate from 

 fixation or later preparation of the material. X 39. 



78 From a transverse section of a 10 mm. pig embryo through the region of 

 the V ganglion. The increased vascularity of the mesenchyme above the roof 

 plate together with the enormous amount of coagulated cerebro-spinal fluid 

 (S.C.F.) in the ventricle are evidences of the factors which have produced the 

 increased expansion noticed in the roof of the ventricle. Also at this stage the 

 pontine flexure has increased to such an extent that its action on a fourth ventri- 

 cle full of cerebro-spinal fluid, itself under a moderate pressure, would produce 

 a further expansion of the roof plate. As in the preceding sections the roof plate 

 has suffered a collapse in the preparation of the material. X 39. 



74 



