26 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



Grenzvene) ; the large tributary, draining the lateral surface of the cerebrum, that 

 empties into it, he calls the lateral telencephalic vein, of which there may be several. 

 Markowski describes the anterior marginal veins of the two sides as extending 

 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 2, 3, and 4 it can be seen that there is no sharp line between the sagittal 

 plexus as described by us and the more ventral loops of what was the anterior 

 dural i:)lexus of which it is a part. The anterior marginal vein of Markowski is a 

 part of both of them, as can be plainly seen in figure 26. The discussion regarding 

 the formation of the superior sagittal sinus will be reserved for a subsequent part 

 of this paper. We may, however, point out at this time that the anterior marginal 

 vein of Markowski is apparently not a definite vein, but rather a constantly changing 

 channel. What we find is that the more anterior loops of the tentorial plexus are 

 constantly dropping out and are replaced by the development of the more caudal 

 channels. By comparing figures 3 and 4 we can see this change occurring. Our 

 interpretation of the condition found in figure 4 is that what had been a larger 

 channel along the cerebral margin of the tentorial plexus is now dwindling.into a 

 small mesh, whereas the main blood-stream forms for itself a new course in a more 

 caudal loop of the plexus. 



In this connection it may be pointed out that "migration of veins" may occur 

 in at least two ways. There may be a passive change in position or direction of 

 the endothelial tube itself, due to mechanical causes arising from alterations in its 

 environment; this is illustrated by the sigmoid portion of the transverse sinus 

 and its change in form in the later stages (embryos more than 20 mm. long). On 



Direclionof ^p.V?-- -- 



F'«uRE 5. migrcrtion.- 



Camera-Iucida sketch of a portion of the right transverse sinus in 



a human embrj'O 27..5 mm. long (Carnegie Collection, No. 1458). ■'/ ^ JL 



It illustrates the manner of channel-migration in the loops of , ,'//y/ -Dwindling channel, 



the tentorial plexus, whereby the main channel follows a course 

 in adaptation to the caudal growth of the cerebral hemisphere. 

 While this process is under way there are found along the cere- 

 bral margin the slender remnants of discarded channels. 



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 chan- 

 nel. The embryonic plexiform character of the veins in the region of the tentorium 

 is especially favorable for this procedure, and we find this type of alteration in 

 the blood-channels repeatedly illustrated in this region. In other words, under 

 migration of veins we are to distinguish between passive migration (where there is a 

 change in position due to some flexion or traction on the vein-wall itself) and spon- 

 taneous migration (where there is a change in position of the blood-stream only), and 

 where, by a process of what might be called circumfluent anastomosis or anasto- 

 motic progression, the blood-stream develops a new channel in the adjacent loops 

 of the plexus, with a corresponding dwindling of the previously used loop, as is 

 illustrated in figiu-e 5. From the observations of Evans (1912) on the ventral 



