170 GEORGE L. STREETER 



described by Evans, have fused across the middle line and have 

 formed a longitudinal network, in the meshes of which an asym- 

 metrical channel is finallj' established, that we can speak of a 

 superior sagittal sinus. 



Owing to the growth of the cerebral hemispheres in 20 mm. 

 embryos, the dural tissue lying between them begins to take on the 

 form of the falx cerebri. It is in this loose dural tissue that the 

 meshes of the sagittal plexus are found. At this time it can be 

 seen that a larger channel is opening along the dorsal mid-line 

 that will form the superior sagittal sinus, and connected with it 

 by anastomosing loops, is a more ventrally situated large chan- 

 nel that constitutes the 'sinus rectus.' This latter extends for- 

 ward and drains the lower part of the falx. It has two converg- 

 ing limbs in front that drain the chorioidal masses of the hemi- 

 spheres. This and the more superficial channels in this embryo 

 drain chiefly to the right side. Smaller anastomosing loops 

 connect also with the plexus in the region of the left transverse 

 sinus. 



On coming to embryos 50 mm. long (fig. 9, No. 458, 54 mm. 

 Carnegie Collection) we find that here the superior sagittal sinus 

 is established, at least in part. In its cephalic portion there is a 

 large characteristic channel, lacking only the dural connective 

 tissue investment to make it an adult type. In its more caudal 

 portion it still exhibits a plexiform character that indicates its 

 transitional state. Upon comparison of a number of series the 

 writer is led to interpret the formation of a single channel as the 

 outcome of more than one process ; in some segments there seems 

 to be the selection of a favorable loop of the plexus which enlarges 

 and becomes the main channel, and in other segments there is 

 apparently an enlargement of two or more collateral loops which 

 subsequently fuse into a more or less common channel. Both 

 processes are apparently represented in figure 9. It is to be 

 expected that we shall find a considerable variation in this 

 in different brains. 



No attempt was made to study the histological changes that 

 occur in the completion of the superior sagittal sinus, nor of the 

 cavernous sinus. These involve d'etails with which the present 



