VENOUS SINUSES OF THE DURA MATER 163 



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 spontaneous migration, in which there is a change in position 

 of the blood stream only, where by a process of what might be 

 called circumfluent anastomosis or anastomotic progression, the 

 blood stream develops a new channel in the adjacent loops of the 

 plexus with a corresponding dwindling of the previously used 

 loop. 



The lateral telencephalic veins of Markowski apparently cor- 

 respond to the inferior cerebral veins of the adult, so we shall 

 label them in that way. Though emptying into the dural system 

 they develop their course through the intradural membranes and 

 become typical cerebral veins. It is interesting to note that in 

 the 21 mm. embryo certain definite topographical points in the 

 transverse sinus are already determined, namely, the jugular 

 foramen, the location of the endolymphatic sac, the point of entry 

 of the superior petrosal sinus and of the inferior cerebral veins. 

 Thus we see that more than half the sinus is already established 

 and that it is the terminal or jugular portion that is established 

 first. The remainder of the sinus is relatively late in assuming 

 a permanent form, which is doubtless the result of the prolonged 

 period of growth of the cerebrum, making a continued adjustment 

 of the tentorial plexus necessary. Even in embryos 50 mm. long 

 which we shall now proceed to examine, the proximal end of this 

 sinus is still in the formative stage. 



5. Human embryos 50 mm. long 



To cover the period of embryos about 50 mm. long the writer 

 examined three series belonging to the Carnegie Collection: No. 

 84, 50 mm. transverse; No. 96, 50 nam. sagittal; and No. 448, 52 

 mm. sagittal-injected. There was also an embryo of about the 

 same age (No. 458, 54 mm.) that had been injected with India ink. 

 The head was removed and partially dissected, and then cleared 

 after the Spalteholz method. This gave excellent total views of 



