OF THE BRAIN OF THE HUMAN EMBRYO. 



25 



On comparing embryos 21 mm. long with those 18 mm. long two character- 

 istic changes are observed in the pattern of the anterior dural plexus at this time. 

 (Comjjare figs. 3 and 26 with fig. 2). In the first place, the anterior dural plexus 

 annexes itself to the middle dural plexus and drains backward through this into the 

 newly estabhshed channel dorsal to the otic capsule. We will therefore, from now 

 on, refer to the combined anterior and middle dural plexuses as the tentorial plexus, 

 on the basis of its distribution, which is probably a more satisfactory terminology 

 than to group both of them under the designation anterior dural plexus, as was 

 done in the former paper (Ptreeter 1915). In the second place, there is differ- 

 entiated along the margin of the cerebrum and between the hemispheres a sub- 

 division of this ))lexus that is eventually to constitute the superior sagittal sinus 

 (marked plexus sagittalis in figs. 3, 4, and 26). 



PLEXUS TENTORII 



PLEXUS 



SAGITTALIS 



SIN. TRANSVERSUS 



V. OPTHAL. 



SIN. CAVERNOSUS 



Figure 4. 



Profile reconstruction of the main dural veins in a human embryo 24 mm. long (Carnegie Col- 

 lection, No. 6.32). The sigmoid portion of the transverse sinus can be identified as that part 

 between the point of entry of the superior petrosal sinus and the jugular foramen (marked X). 

 Enlarged about 5 diameters. 



Examination of jihotographs and sketches of embryos of about this age shows 

 that there is a tendency to the formation of a larger channel along the anterior 

 margin of the tentorial plexus — that is, along the caudal margin of the cerebrum. 

 This was designated by Markowski (1911) as the anterior marginal vein (vordere 



