. Carnegie Institution of Washington publication. OF THE BRAIN OF THE HUMAN EMBRYO. 31 there is everywhere a fine anastomosing network. Also, the channels do not lie all in the same plane. Furthermore, it is to be noted that there exists in embryos of the same age a considerable variation in the pattern formed by these channels. The three specimens selected, however, may be regarded as illustrating fairly definite stages in this transformation. In figure 7 is shown a dorsal view of the head of the same embryo previously shown in figure 1 (Carnegie Collection, No. 940, mm. long). It is abo
. Carnegie Institution of Washington publication. OF THE BRAIN OF THE HUMAN EMBRYO. 31 there is everywhere a fine anastomosing network. Also, the channels do not lie all in the same plane. Furthermore, it is to be noted that there exists in embryos of the same age a considerable variation in the pattern formed by these channels. The three specimens selected, however, may be regarded as illustrating fairly definite stages in this transformation. In figure 7 is shown a dorsal view of the head of the same embryo previously shown in figure 1 (Carnegie Collection, No. 940, mm. long). It is about the same age as the embryos shown in figures 24 and 25. Here we find the sagittal plexus represented in its simplest form. It will be noted that it possesses two characteristic features: In the first place, there is a tendency to an enlargement of TELENCEPHALON SIN. SAGITTALIS DIENCEPH. MESENCEPHALON. MESENCEPHALON CEREBELLUM' 8 9 Figs. 7, 8, and 9.—Three stages in the formation of the sagittal plexus, showing its asymmetrical character and its conversion into the superior sagittal sinus. Draining into it from below is the drainage-channel from the <;horioidal bodies that ia later known as the straight sinus. The channels marked x are interpreted as undergoing retrogression, being replaced by more caudal channels. Large dural channels do not persist in the areas covering the cerebral hemispheres; they are found only in the loose embryonic tissue filling the spaces and fissures that lie between the different subdivisions of the brain. Figure 7 is a vertex view of a human embryo mm. long (Carnegie Collection. No. 940). Figure 8 is a ver- tex view of an embryo 20 mm. long (Carnegie Collection, No. 349). Figure 9 is a drawing of an injected and cleared specimen 54 mm. long (Carnegie Collection, No. 458) reversed. certain portions of the plexus, irrespective of a continuous channel. We thus have a series of small lakelets connected by narrow channels. A definite single
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