THE DEVELOPMENT OF THE CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



BY LEWIS H. WEED. 



I. INTRODUCTORY. 



Probably no field in embryology has been less explored than that relating to 

 the meninges. Our knowledge of the transformation of the perimedullary mesen- 

 chyme into the three fully developed membranes about the cerebro-spinal axis has 

 been largely of a crude sort, with gross generalities based on inexact or incomplete 

 evidence. The present work was undertaken in the hope that by a study of the 

 various stages in the development of the cerebro-spinal spaces there might be 

 gained some knowledge which would afford a basis for a conception of this dynamic 

 metamorphosis. 



Many of the problems centering around the development of the meningeal 

 spaces have recently been expounded by Gushing W . * Not only dowelack knowledge 

 as to the method of differentiation of the primitive mesenchyme, but we know little 

 about the establishment of the circulation of the cerebro-spinal fluid. When do the 

 chorioid plexuses begin to secrete? When does the venous absorption of the fluid 

 take place? When does the perivascular system begin to remove waste products 

 from the cerebral tissue? And also, what factors play a part in the formation of 

 the subarachnoid and subdural spaces? 



These questions, some of which it is hoped the present study will answer, relate 

 to the field of physiological anatomy. Consideration of the subject, however, serves 

 to convince one that they must be investigated coincidently with the stages of 

 morphological differentiation ; for it may readily be conceived that the physiological 

 use of the meningeal spaces may precede any morphological differentiation of the 

 three membranes, nor indeed is it unlikely that one of the active causative factors 

 in the metamorphosis concerns this filling of the mesenchyme about the nervous 

 system with fluid. 



This study, therefore, has been anatomical, but with a broader scope than 

 purely morphological studies would have afforded. Not only has it dealt with the 

 morphological differentiations about the nervous system, but throughout the inves- 

 tigation the relationship of these structures to the possible presence of cerebro- 

 spinal fluid has been considered. As the problem developed it was projected more 

 and more into the difficult realm of "tissue spaces." Interest in these spaces 

 largely concerned their physiology, but many points of correspondence between 

 structure and function were found. 



In some measure this work is a development of an earlier study of some of the 

 anatomical and physiological problems of the cerebro-spinal fluid, carried out in the 

 laboratory of Dr. Harvey Gushing at the Harvard Medical School. 



*The figures in parentheses refer to the bibliography at the end of this paper. 



