THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 

 OF THE BRAIN OF THE HUMAN EMBRYO. 



By George L. Streeteb. 



INTRODUCTION. 



One of the most striking features in the development of the blood-vessels of 

 the head is the clear way in which they demonstrate the embryological principle 

 of what may be termed integrative development. It is quite evident that the 

 vascular apparatus does not independently and by itself "unfold" into the adult 

 pattern. On the contrary, it reacts continuously in a most sensitive way to the 

 factors of its environment, the pattern in the adult being the result of the sum 

 of the environmental influences that have played upon it throughout the embry- 

 onic period. We thus find that this apparatus is continuously adequate and 

 complete for the structures as they exist at any particular stage; as the environ- 

 mental structures progressively change, the vascular apparatus also changes and 

 thereby is always adapted to the newer conditions. Furthermore, there are no 

 apparent ulterior i^reparations at any time for the supply and drainage of other 

 structures which have not yet made their appearance. For each stage it is an 

 efficient and complete going-mechanism, apparently uninfluenced by the nature of 

 its subsequent morphology. 



With these factors in mind, one can better understand the architectural 

 arrangements of the vascular system of the head that appear in different periods of 

 development. In the primordial or precirculatory period the vessels that then 

 exist are engaged principally in growth and in the elaboration of a plexus, and 

 their form then is little influenced by conditions that would favor the circulation 

 of the contained fluid. As the cu-culatory flow of the blood becomes estabUshed 

 we find that the vascular plexus responds by conforming to the hydrodynamic 

 requirements, becoming adequately adapted to the form of the neural tube as then 

 existing, with favorably situated aortic feeders and simple and direct drainage 

 channels. As the brain becomes more complicated, and as the skull-membranes 

 form, there occur, step by step, the necessary adaptations on the part of the blood- 

 vessels. Finally, when the permanent form is attained, the vessels lose their transi- 

 tory character and develop permanent and more highly difi"erentiated walls, properly 

 suited to the adult functional requirements. 



It is possible to subdivide the development of the blood-vessels of the brain 

 into five successive periods, each showing special adaptations to their changing 

 environmental conditions. To facilitate the description of this process an arbitrary 

 order of that kind will be adopted in this paper. During the first of these five 

 periods there are established the primordial endothelial blood-containing channels, 

 which are neither arteries nor veins, but constitute the source from which all the 

 arteries, veins, and capillaries of the brain are derived. These primordial blood- 



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