. Contributions to embryology. Embryology. 32 DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM Evans, 1912, figs. 399 and 400). According to him it is thus originally paired and l)ilaterally symmetrical. According to the present writer, it is not until later that we can speak of a superior sagittal sinus. It is not until the plexuses, described by Evans, have anastomosed across the median line and have formed a longitudinal network in the meshes of which an asymmetrical channel is finally established, that we can speak of a superior sagittal sinus. In some cases two or more larger longitudinal
. Contributions to embryology. Embryology. 32 DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM Evans, 1912, figs. 399 and 400). According to him it is thus originally paired and l)ilaterally symmetrical. According to the present writer, it is not until later that we can speak of a superior sagittal sinus. It is not until the plexuses, described by Evans, have anastomosed across the median line and have formed a longitudinal network in the meshes of which an asymmetrical channel is finally established, that we can speak of a superior sagittal sinus. In some cases two or more larger longitudinal channels are formed in the mesh, as is shown in figure 10; but in such cases they are not strictly bilaterally symmetrical. Owing to the gi-owth of the cerebral hemispheres in 20 mm. embryos, there is formed a well-marked cerebral longitudinal fissure which is occujiied by embryonic tissue. This rapidly takes the form of the adult falx cerebri. It is in the dorsal part of this loose dural tissue that the meshes of the sagittal plexus are found. TELENCEPHALON. FlGlRE 10. Camera-lucida sketch of the vertex of the head of a human embryo mm. long (Carnegie Collection, No. 1458), in which the blood-vessels were conspic- uous through a natural injection. It is the exception where two channels are found so nearly the same size. From a single specimen, like this, one might incorrectly gain the impression that the superior sagittal sinus is at first bilat- erally symmetrical. Even here, how- ever, it can be seen that both of the larger channels drain cavidally to the right .side. The minute anastomosing loops that connect the two main chan- nels with each other and with the chan- nels in the more ventral paits of the falx are not shown. On each side can be seen the serrated upper margin of the advancing capillary plexus of the skin. Above this margin the skin is as yet nonvascular. On account of the dilatation of these vessels along this advancing edge the margin is conspic- uous.
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