148 GEORGE L. STEEETER 



by the head vein its direct connection with the aorta is broken 

 and a circulation is thus estabhshed from the primary brain 

 plexus backward through the cardinal plexus to the venous end 

 of the heart. In the formation of this circulation we are thus 

 deahng with two capillary groups; the cephalic one, belonging 

 intrinsically to the head, and the caudal one, belonging to the 

 cardinal system. 



Very soon after the establishment of this circulation in the 

 head it can be seen that the single vessel derived from the fusion 

 of dorsal sprouts of the aorta, which we have described as devel- 

 oping caudalward alongside of the brain, has become the middle 

 segment of a prominent longitudinal channel that extends all the 

 way from the optic stalk to the duct of Cuvier. The cephalic 

 segment of this channel and its tributaries are formed -in the 

 more superficial loops of the primary brain plexus; its caudal 

 segment forms in the cardinal plexus and constitutes the anterior 

 cardinal vein, eventually the internal jugular vein. The whole 

 channel is designated as the 'primary head vein'' and with its estab- 

 lishment we may regard the primitive arrangement of the drainage 

 of the brain as completed. Essentially, it consists of a sheet of 

 capillaries that nearly everywhere surrounds the brain tube, and 

 this capillary sheet is drained by many irregularly placed anas- 

 tomosing loops into the more superficially placed primary head 

 vein, which runs along the side of the hind-brain and empties 

 finally in the duct of Cuvier at the venous end of the heart. 



STAGES OF DEVELOPMENT OF THE DURAL VEINS 



1. Human embryos 4 mm. long 



It is this primar}^ arrangement of the drainage of the head, 

 which we have just described, that exists in 4 mm. human em- 

 bryos, and this is the earliest stage examined in connection with 

 the present study. In figure 1 is shown a profile reconstruction of 

 such an embryo (No. 588, 4 mm. long, Carnegie Collection). 

 This is slightly younger than the stage shown by Mall ('05) in his 

 figure 3. The conditions in the two, however, are very similar and 

 the relations of the main vein are the same. In our figure the 



