Journal of comparative neurology . plate. It should also be recordedthat a few blood vessels have entered the outer surface of the medulla. As inthe previous series the sections have suffered a collapse of the roof plate fromfixation or later preparation of the material. X 39. 78 From a transverse section of a 10 mm. pig embryo through the region ofthe V ganglion. The increased vascularity of the mesenchyme above the roofplate together with the enormous amount of coagulated cerebro-spinal fluid() in the ventricle are evidences of the factors which have produced theincreased expansion not
Journal of comparative neurology . plate. It should also be recordedthat a few blood vessels have entered the outer surface of the medulla. As inthe previous series the sections have suffered a collapse of the roof plate fromfixation or later preparation of the material. X 39. 78 From a transverse section of a 10 mm. pig embryo through the region ofthe V ganglion. The increased vascularity of the mesenchyme above the roofplate together with the enormous amount of coagulated cerebro-spinal fluid() in the ventricle are evidences of the factors which have produced theincreased expansion noticed in the roof of the ventricle. Also at this stage thepontine 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 producea further expansion of the roof plate. As in the preceding sections the roof platehas suffered a collapse in the preparation of the material. X 39. 74 SPINAL CORD AND MEDULLA OF CYCLOSTOMES WILLIAM F. .VLLEN C^.F. 71) and SO Iwo transviisi- sections tlinmnh tlu- anti-riur ami |iii>icimr i-iiilsof the fourth ventricle from a 14 mm. frontal series of a pig. .\ more advancedstage in the development of the chorioid plexus together with a more pronouncedpontine flexure has produced a much larger fourth ventricle and expanded roofplate than is shown in the previous series (fig. 78). The thickening of the lateralwalls of the medulla is taking place as it did in Petromyzon and Squalus, but thegreater expansion of the ventricle in the pig (fig. 79) has prevented the wallsfrom fusing ventrally. Nevertheless, the thickening of the ventral portion ofthe lateral plates would increase the pressure of the cerebro-spinal fluid. X 25and 30. ABBREVI.^TIONS , roof plate expansion , cerebro-spinal fluid , Gasserian or semilunar ganglion 4 v., fourth ventricle , vagus root , blood vessel , cerebro-spinal fluid , layer of ependymal nuclei ,
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