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, 13.8 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 13.8 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 superior 

 sagittal sinus can not yet be said to exist. In the second place, the plexus is dis- 

 tinctly asymmetrical and shows a tendency to drain more freely to one side than 

 the other, in this case to the right. 



A more definite and simpler channel system is found in 20 mm. embryos, an 

 example of which is shown in figure 8 (Carnegie Collection, No. 349). Here 

 one might possibly speak of a superior sagittal sinus. The channels, however, 

 are still in the form of a plexus, and hence the term plexus sagittalis is retained. 

 This view regarding the early identity of the superior sagittal sinus differs from 

 that given by Evans, who pictures the primitive capillary plexus creeping up on 

 each side of the forebrain in 8 mm. pig embryos. A portion of the dorsal margin of 

 this plexus he labels as the primitive superior sagittal sinus (Evans, 1909, fig. 156; 



SIN. 

 TRANS. 



