. Carnegie Institution of Washington publication. OF THK BRAIN 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 colla
. Carnegie Institution of Washington publication. OF THK BRAIN 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. 11 12. Fii:. 11.—Scption showins the sagittal plexus in a human embryo It iiiiii. loii!^ (Carnegie Collection. No. 940. slide , row 3, section 1). The section shows the falciform area, the hemispheres lieing retracted from its lateral margins. It will be noted that there are two main plexiform vascular sheets—a superficial one near the skin and a deeper one directly against the brain-wall, the latter draining into the former by anastomosing loops. The superior sagittal sinus in the meshes of the superficial plexus, and the straight .sinus develops in the meshes of the deep plexus over the area corresponding to the third ventricle. Fig. 12.—Section showing the sagittal plexus in a human embryo 21 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
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