. Contributions to embryology. Embryology. OF THE BRAIX OF THE HUMAN EMBRYO. 33 connective-tissue investment to make it an adult type. In its more caudal portion it still exhibits a plexiform character that indicates its transitional state. Upon comparison of a number of series, the writer is led to interpret the formation of a single channel as the outcome of more than one process; in some segments there seems to be the selection of a favorable loop of the plexus which enlarges and be- comes the main channel, and in other segments there is apparently an enlargement of two or more collateral l
. Contributions to embryology. Embryology. OF THE BRAIX OF THE HUMAN EMBRYO. 33 connective-tissue investment to make it an adult type. In its more caudal portion it still exhibits a plexiform character that indicates its transitional state. Upon comparison of a number of series, the writer is led to interpret the formation of a single channel as the outcome of more than one process; in some segments there seems to be the selection of a favorable loop of the plexus which enlarges and be- comes the main channel, and in other segments there is apparently an enlargement of two or more collateral loops which subsequently fuse into a more or less common channel. Both processes are apparently represented in figure 9. It is to be expected that we will find a considerable variation in this respect in different brains. In figure 10 is shown a specimen which is about the same age as that shown in figure 8. In this case two collateral channels of about equal size have formed, both draining, however, to the same side. ^^V. /^& k'''<^ B w^Il . J/^ li;.^:S llP4UAAlMlflHi^^^Hk'<^Li£¥.:. Fig. 11.—.Section showing the sagittal plexus in a human embryo 14 mm. I"!,:; ' : j;' I'ollection. Xn. '.tv\. 'o21 mm. long (Carnegie Collection, No. 460, slide 11, row 3, section 4), injected with India ink. The arrangements are similar, but more advanced than those shown in figure 11. The straight sinus can be recognized. The superior sagittal sinus still has the form of a bilaterally asymmetrical plexus from which a finer meshwork extends into the loose falciform tissueinter\ening between the two hemispheres and anasto- moses with the straight sinus. On the left side a characteristic communication can be seen connecting the deep cerebral plexus with the coarser sagittal plexus above. No attempt was made to study the histological changes that occur in the completion of the superior sagittal sinus, or of the cavernous sinus. These involve details with which the present paper is n
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