34 



DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



represents the point at which this caudal development reaches its completion — or, 

 in other words, is a remnant of the embryonic tentorial plexus and usually retains 

 a trace of the plexiform character that is found throughout the embryonic stages. 

 It is interesting to note that the asymmetry of the superior sagittal sinus 

 expresses itself in the embryo as well as in the adult by a tendency to drain more to 

 one side of the head than to the other. This becomes established by the time the 

 embryo is 20 mm. long. The drainage is preponderantly toward the right side. 

 It happens that in figure 9 the main drainage was in reaUty toward the left side. 

 In reproducing the sketch the figure was reversed right for left, in order to facilitate 

 its comparison with figures 7 and 8. In the accompanying table is given a list 

 of embryos which were examined as to this point, and it will be seen that of 18 

 specimens all but 2 drained predominantly toward the right side, that is, about 89 

 per cent. In order that account should be taken of the artificial element introduced 

 in those specimens where the vascular system had been injected with coloring 

 matter, such specimens are indicated in the table by an asterisk. No explanation 

 has thus far been reached to explain this interesting asymmetry. The drainage 

 of the straight sinus could not be determined as well in the younger stages, and 

 there were not enough of the older stages upon which to base an average. A 

 similar asymmetry might be expected here. 



Superior Sagittal Sinus. 



* Injected specimens. 



SUMMARY. 



In describing the development of the blood-vessels of the brain the process 

 has been subdivided into the following five arbitrary periods: (1) angioblastic 

 period, in which the primordial vascular system becomes established; (2) resolution 

 of the primordial system into arteries, veins, and capillaries, and the estabhshment 

 of the primary type of circulation; (3) cleavage of the vascular system of the head 

 into the external, dural, and cerebral layers; (4) adjustments of vascular channels, 

 due chiefly to growth and change in form of the otic capsule and the brain; and 

 (5) completion of histological differentiation of the walls of the vessels. Aside from 

 adding perhaps more emphasis to the morphological aspects of the precirculatory 

 type of the vascular system, the contributions of this paper are concerned only 

 with the third and fourth developmental periods, and even there they are more or 

 less restricted to the main drainage-channels and their gradual metamorphosis into 

 the adult dural veins and sinuses. It has been possible to present a rather com- 

 plete series of stages from which the essential factors in this process can be clearly 



