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



due to normal or experimental agencies) causes a marked extraventricular spread 

 seems firmly established. For the colloidal suspensions (particularly the protein 

 of the normal ventricular fluid) a slower process of diffusion and filtration seems the 

 probable agency for passing the ventricular colloids into the subarachnoid spaces. 



That the results obtained by the method of replacement were not solely due 

 to diffusion, but represent a filling of the physiological extent of the cerebro-spinal 

 spaces, has been shown in many ways, but probably the chief argument against such 

 a view is that wholly similar extensions of the foreign solution may be obtained by 

 injections under mild pressures from a syringe; with increasing pressures these 

 injections show the same type of spread, but always in a smaller embryo than the 

 replacement method demonstrates as the standard for a given stage of the extension. 

 The results recorded in the foregoing pages indicate also that suspensions (india ink) 

 and true solutions (when powerful precipitants) are valuable only for affording 

 comparisons in problems concerning the normal processes of absorption. 



Of interest in any discussion of the results of injections into the perispinal spaces 

 or into the spinal central canal are the findings in regard to the perineural spaces. 

 It is possible to inject such spaces around each of the segmental nerves, but only 

 when the pressures of injections are extreme. In no case, however, were such 

 injections found to enter the lymphatic system a finding in accord with the obser- 

 vations of Reford and Sabin. The physiological importance of these spaces in the 

 adult is probably great, but the same methods of demonstration (with carefully 

 controlled pressures) which suffice in the adult are unavailing in the embryo. 



The origin of the three meninges from the perimedullary mesenchyme is well 

 established. His, Kolliker, Sterzi, Farrar, and others have placed this conception 

 on a very firm basis. Most of the investigators have been concerned with the 

 differentiation of the spinal meninges, while the observations here reported have been 

 concerned solely with the cranial portion of these membranes. In general, the same 

 phenomena in the transformation of the primitive periaxial mesenchyme as recorded 

 by these earlier workers may be found in the cranium. The division of the primitive 

 mesenchyme by a secondary condensation, a view advanced chiefly by Salvi, seems 

 well supported. The findings in the cranium are in accord with this conception; 

 the outer portion of this primitive meninx becomes the dura mater, the inner forms 

 both the pia and arachnoid. The processes in the formation of the arachnoid are, 

 however, quite diversified and concern both the formation of the subarachnoid 

 spaces and the outer membrane of the arachnoid. 



Out of the rather loose-meshed periaxial mesenchyme, the subarachnoid spaces 

 develop. The process concerns the transformation of the small "tissue spaces" 

 of this mesenchyme into the larger subarachnoid channels, which are interrupted by 

 the well-known arachnoid trabeculse. Well-marked stages in this metamorphosis, 

 which begins in the basis cranii, can be made out. The first appearance of a differ- 

 entiation is seen in the gradual increase in the size of the mesenchymal mesh. This 

 is closely associated with an increased amount of an albuminous coagulum which in 

 a measure fills the larger interstices. Following this initial dilatation of the spaces 



