OF THK BRAIN OF THE HUMAN EMBRYO. 



33 



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 be- 

 comes 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 will find a considerable variation in this respect in different 

 brains. In figure 10 is shown a specimen which is about the same age as that 

 shown in figure 8. In this case two collateral channels of about equal size have 

 formed, both draining, however, to the same side. 



11 12 



Fii:. 11. — Scption showins the sagittal plexus in a human embryo It iiiiii. loii!^ (Carnegie Collection. No. 940. slide 1.5, row 

 3, section 1). The section shows the falciform area, the hemispheres lieing retracted from its lateral margins. It will 

 be noted that there are two main plexiform vascular sheets — a superficial one near the skin and a deeper one directly 

 against the brain-wall, the latter draining into the former by anastomosing loops. The superior sagittal sinus develop.s 

 in the meshes of the superficial plexus, and the straight .sinus develops in the meshes of the deep plexus over the area 

 corresponding to the third ventricle. 



Fig. 12. — Section showing the sagittal plexus in a human embryo 21 mm. long (Carnegie Collection, No. 460, slide 11, row 3, 

 section 4), injected with India ink. The arrangements are similar, but more advanced than those shown in figure 11. 

 The straight sinus can be recognized. The superior sagittal sinus still has the form of a bilaterally asymmetrical plexus 

 from which a finer mesh work extends into the loose falciform tis.sue intervening between the two hemispheres and anasto- 

 moses with the straight sinus. On the left side a characteristic communication can be seen connecting the deep cerebral 

 plexus with the coarser sagittal plexus above. 



No attempt was made to study the histological changes that occur in the 

 completion of the superior sagittal sinus, or of the cavernous sinus. These involve 

 details with which the present paper is not concerned. The caudal ward growth, 

 however, of the superior sagittal sinus in adjustment to the corresponding growth 

 of the hemispheres is of interest in our general problem. By comparing figures 7, 

 8, and 9 it can at once be seen that this caudal development is accomplished at the 

 expense of the meshes of the tentorial plexus, in which process the transverse and 

 straight sinuses also take part. These channels gradually obtain- a more caudal 

 course by what we have already described as spontaneous migration. The channel 

 repeatedly shifts into a more caudal loop of the plexus, the new loop enlarging and 

 the old loop dwindling. The veins marked X in figures 8 and 9 may thus be inter- 

 preted as discarded channels. The eventual confluens sinuum (torcular Herophili) 



