168 ■ GEORGE L. STREETER 



in between the developing hemispheres. It is in the meshes of 

 this plexus that we find the beginning of the superior sagittal 

 sinus; and the principal steps in its transformation can be seen 

 by comparing figures 7, 8 and 9. Sketches like these necessarily 

 have to be simphfied, and on examining them it should be re- 

 membered that only the larger channels are shown and in between 

 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 consider- 

 able 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 em- 

 bryo previously shown in figure 2 (No. 940, 13.8 mm. long, Car- 

 negie Collection). 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 certain portions of the plexus, irrespective of a 

 continuous channel; we thus have a series of small lakelets con- 

 nected by narrow channels; a definite single superior sagittal sinus 

 cannot yet be said to exist. In the second place, the plexus is 

 distinctly 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 (No. 349, 20 

 mm. long, Carnegie Collection). 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 re- 

 tained. 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 fore- 

 brain in 8 mm. pig embryos. A portion of the dorsal margin 

 of this plexus he labels as the 'primitive superior sagittal sinus' 

 (Evans '09, fig. 15 b; Evans '12, figs. 399-400). According to 

 him, it is thus originally paired and bilaterally 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, 



