76 GEORGE L. STREETER 



vein, it unites with the middle dural plexus and they both drain 

 into the posterior dural plexus and through it into the internal 

 jugular vein. Due to these alterations in the drainage of the 

 anterior and middle dural plexuses the greater part of the pri- 

 mary head vein disappears and we find it replaced by the more 

 dorsally situated channel that is to become the transverse sinus. 

 This channel forms in a groove in the dorsal margin of the otic 

 capsule. Topographically it passes longitudinally in the space 

 between the two vertical canals and the endolymphatic sac. The 

 general relation of these structures is shown in figures 1 and 2 

 which are reproduced from the paper previously referred to 

 (Streeter '15). The canals are separated from the sinus by their 

 cartilagenous envelope. The endolymphatic sac, however, like 

 the transverse sinus itself does not become encased by cartilage 

 and lies against the median wall of the latter, separated from it 

 only by a small amount of loose embryonic connective tissue in 

 which both are embedded. This close relation which becomes 

 established between the endolymphatic sac and the transverse 

 sinus in 18 mm. embryos, continues as a permanent condition. 

 At first (fig. 1) when the endolymphatic sac has a vertical posi- 

 tion, it completely overlaps the median surface of the sinus. 

 Subsequently as the cranium enlarges, this part of its wall is 

 crowded outward and downward into a more horizontal position 

 and partakes in the formation of the floor of the posterior cerebral 

 fossa. We then find the endolymphatic sac resting on the dorsal 

 surface of the sinus and furthermore the sinus becomes relatively 

 larger than the sac and is then only partly overlapped by the 

 latter. 



Though closely related to the chorioidal membrane of the lat- 

 eral recess, the endolymphatic sac becomes more and more clearl}^ 

 separated from it as the dural and arachnoidal tissues become- 

 differentiated. On the other hand, though resting against the 

 transverse sinus, there is a scant amount of loose embryonic 

 connective tissue separating the two. Running through the 

 meshes of this connective tissue can be seen blood capillaries 

 that form a plexus which empties into the transverse sinus. This 

 plexus anastomoses with the vessels of the labyrinth bj'^ com- 



