162 GEORGE L. STREETER 



larger channel along the anterior margin of the anterior dural 

 plexus, that is, along the caudal margin of the cerebrum. This 

 was designated by Markowski ('11) as the ' anterior marginal vein' 

 (vordere Grenzvene) and the large tributary, draining the lateral 

 surface of the cerebrum, that empties into it he calls the ' lateral 

 telencephahc vein,' of which there may be several. Markowski 

 describes the anterior marginal veins of the two sides as ex- 

 tending forward and toward the median line and uniting in the 

 formation of a plexus out of which is to be derived eventually 

 the superior sagittal sinus. From examination of figures 3, 4 and 

 5 it can be seen that there is no sharp line between the sagittal 

 plexus as described by us and the more ventral loops of the 

 anterior dural plexus of which it is a part. The 'anterior mar- 

 ginal vein' of Markowski is a part of both of them. The dis- 

 cussion regarding the formation of the superior sagittal sinus 

 will be reserved for a subsequent part of this paper. We may 

 point out at this time, however, that the anterior marginal vein 

 of Markowski apparently is not a definite vein but rather a con- 

 stantly changing channel. What we find is that the more an- 

 terior loops of the anterior dural plexus are constantly dropping 

 out and are replaced by the development of the more caudal 

 channels. By comparing figures 4 and 5 we can see this change 

 occurring. Our interpretation of the co"iidition found in figure 5 

 is that what had been a larger channel along the cerebral margin 

 of the dural plexus is now dwindling into a small mesh, while the 

 main blood stream forms for itself a new course in a more caudal 

 loop of the plexus. In this connection it is well to remember that 

 migration of veins may occur in two ways. There may be a pas- 

 sive change in position or direction of the endothelial tube itself, 

 due to mechanical causes arising from alterations in its environ- 

 ment. This is illustrated by the sigmoid portion of the trans- 

 verse sinus and its change in form in the later stages (embryos 

 more than 20 mm. long). On the other hand, a vein may change 

 its position by forming or adopting a new endothelial channel and 

 at the same time relinquishing its original endothelial channel. 

 The embryonic plexiform character of the veins in the region of 

 the tentorium is especially favorable for this procedure and we find 



