OF THE BRAIN OF THE HUMAN EMBRYO. 11 



vessels of the brain and its membranes are derived. Sabin has suggested a greater 

 restriction in the use of the terms "artery" and "vein" and has warned against 

 making a too early identification of the adult vessels in the embryo. The 

 primordial vascular system of the brain as seen in the chick of 15 somites consists 

 morphologically of a sprouting meshwork rather than a set of definite supply and 

 drainage channels. It is to be considered as a bed of proliferating endothelium 

 rather than as a circulatory apparatus. Even after the blood begins to move 

 through its meshes it is some httle time before the cnculatory function becomes the 

 dominating influence in the determination of its architectural features, and thus, 

 in these early stages of the vascular system, we meet arrangements which, as 

 regards their form, size, and communications, are distinctly inefficient as to the 

 circulatory flow of the contained blood, but are quite characteristic of the germinal 

 period of an endothehal meshwork. Any attempt to distinguish arteries from 

 veins in this primordial system results only in hopeless confusion. 



DIFFERENTIATION OF PRIMORDIAL SYSTEM INTO ARTERIES. VEINS, AND CAPILLARIES. 



Our information regarding the angioblastic period in which the primordial 

 blood-vessels are laid down has been derived mainly from study of chick embryos. 

 The transition into the second developmental period in which the primordial 

 system gradually undergoes resolution into arteries, veins, and capillaries has 

 been demonstrated in mammalian material (pig embryos), and toward the end of 

 the period, as the primary circulation becomes established, the chief features have 

 been described in human embryos. As illustrating these transitions, the reader 

 is referred to figures 394, 395, and 400 of Evans (1912), and figure 1, plate 4, of 

 Sabin (1917a). From the time when the primordial blood-vessel system of the 

 head is first laid down, its endothehal walls undergo active prohferation and a 

 sprouting meshwork extends from it, invading the interval between the ectoderm 

 and the brain-wall. This spreading of the endothelial plexus is more active in 

 some directions than in others. In general it tends to spread toward and over 

 the surface of the neural tube and the nerve structures connected with it. It 

 extends early over the midbram and forebrain and soon encircles the optic stalk. 

 Along the hindbrain the plexus formation is somewhat slower. In fact, in the chick 

 the primordial vessel in this region persists as a simple channel until the embryo 

 has acquired about 29 somites, by which time the growth in the more cephalic 

 region is quite extensive. Persisting so long unchanged, as it does in the chick, 

 it was named the "vena capitis media," in contradistinction to the more laterally 

 situated channel that develops later, the so-called "vena capitis lateraUs." It 

 was shown, however, by Sabin (1917a) that what exists here in the chick at this 

 time is the persisting primordial channel and not a true vein. 



In its earUer stages the prohferating meshwork shows considerable irregularity 

 in the form and size of its constituent channels. Gradually it can be seen that the 

 more superficial loops of the mesh are taking the form of larger and more directly 

 continuous channels. The deeper loops of the mesh flatten out in a more uniform 

 capillary sheet that lies in direct apposition to the brain-wall and its attached 



