SPINAL CORD AND MEDULLA OF CYCLOSTOMES 



WILLIAM F. .VLLEN 



C^.F 



71) and SO I'wo transvi'isi- sections tlinmnh tlu- anti-riur ami |iii>ici mr i-iiils 

 of the fourth ventricle from a 14 mm. frontal series of a pig. .\ more advanced 

 stage in the development of the chorioid plexus together with a more pronounced 

 pontine flexure has produced a much larger fourth ventricle and expanded roof 

 plate than is shown in the previous series (fig. 78). The thickening of the lateral 

 walls of the medulla is taking place as it did in Petromyzon and Squalus, but the 

 greater expansion of the ventricle in the pig (fig. 79) has prevented the walls 

 from fusing ventrally. Nevertheless, the thickening of the ventral portion of 

 the lateral plates would increase the pressure of the cerebro-spinal fluid. X 25 

 and 30. 



ABBREVI.^TIONS 



R.Ex., roof plate expansion 



S.C.F., cerebro-spinal fluid 



V.G., Gasserian or semilunar ganglion 



4 v., fourth ventricle 



X.R., vagus root 



B.V., blood vessel 



C.S.F., cerebro-spinal fluid 



Ep.N., layer of ependymal nuclei 



Mar.L., marginal layer 



M.L., mantle layer 



R.C., embryonic red corpuscle 



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