CHORIOID PLEXUSES AND ELABORATION OF CEREBRO-SPINAL FLUID. 91 



XI. THE CHORIOID PLEXUSES AND THE ELABORATION OF 

 CEREBRO-SPINAL FLUID. 



With the realization that at a definite period in embryonic life, cerebro-spinal 

 fluid passes from the cerebral ventricles into the periaxial spaces, it seemed desirable 

 to ascertain what relationship existed between the developing chorioid plexuses and 

 the elaboration of the fluid; for with the extension of the fluid into the periaxial 

 tissue it becomes obvious that the balance between the development of the intra- 

 ventricular fluid and the volume of the ventricles is destroyed and that more fluid is 

 being elaborated than can be contained within the medullary-canal system. This 

 relationship between the ventricular volume and the production of cerebro-spinal 

 fluid has been described at some length in a preceding section of this communication. 



The determination, then, of the exact role played by the chorioid plexuses in the 

 further extension of the fluid into the periaxial tissue appeared to be of importance, 

 for it could be conceived that the embryonic ependymal cells might be capable of 

 elaborating the excess of fluid. With this purpose in mind the chorioid plexuses 

 were investigated from morphological and cytological standpoints, in the hope that 

 some index might be afforded as to the assumption of function on the part of the 

 developing chorioid plexuses. These methods of study were applied solely to the 

 chorioid plexuses of pig embryos, for it is from them alone that evidence of the period 

 of extraventricular extension of the cerebro-spinal fluid has been obtained. 



THE DEVELOPMENT OF THE CHORIOID PLEXUSES. 



The development of the chorioid plexuses is so well understood that only a very 

 brief outline will be given here. The general scheme of origin of these glandular 

 structures concerns a gradual histological differentiation in certain localities of the 

 ventricular ependyma. The ependyma of the roof of the fourth ventricle thickens 

 along the transverse invagination (plica chorioidea) and then gradually becomes 

 tufted in villous projections into the ventricle, following the ingrowth of a capillary 

 plexus and supporting mesenchyme. This general process of differentiation occurs 

 at first along the lateral portions of the plica; the central portion of the ependyma 

 remains unaffected by the villi even when the tufts have become quite well differ- 

 entiated (fig. 23). 



Quite similar to this process of development of the plexus chorioideus of the 

 fourth ventricle is the differentiation of the other plexuses. The plexus of the third 

 ventricle develops as an infolding of the tela chorioidea of the roof. In every case 

 the process holds of ependymal invagination and subsequent vascularization and 

 suspension by mesenchymal ingrowth. 



The histological differentiation of the ependymal cells into the glandular type 

 of plexus, as first determined by Luschka^ 34 ) and Faivre^ 15 ), is hardly satisfactory 

 as an index of the production of fluid, as the secretory phenomena of the adult cells 

 have not as yet been completely established. The researches of Pet tit and Girard^ 43 ), 

 dealing with the correlation of histological changes in the chorioidal cells and their 

 functional state, first furnished reliable evidence that these cells give rise to cerebro- 



