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



spinal fluid. Since the publication of their investigations in 1900 many workers 

 Meek( 37 ), FindlayW, Pellizzi( 42 ), Mottf 41 ), Hworostuchin' 26 ), EngeK 12 ), and others- 

 have been concerned with this problem and have established on fairly definite bases 

 the relationship of the plexuses to the production of the fluid. The histological 

 appearances of the secretory cells, however, does not rest on incontrovertible ground, 

 as has been stated in a previous paper^ 55 ). 



The process of differentiation of the ependymal cells which form the glandular 

 elements of the chorioid plexuses occurs with the invagination and tufting of these 

 structures. The various stages of transformation from the low type of cubical 

 epithelium constituting the ependymal layer are shown in various figures in this 

 paper. The nuclei of these cells assume basilar positions and the outer zones of the 

 cytoplasm become granular with their greater height. The process is rather a slow 

 one, as might be expected from the fact that the whole villus is gradually enlarging 

 and becoming more and more tufted. 



The histological differentiation of the plexuses need hardly concern us here, 

 except as an index of the assumption of function. The final completion of this 

 change into the adult morphology occurs at a much later stage of development than 

 our evidence indicates for the establishment of a cerebro-spinal circulation. It 

 becomes obvious, then, that the final histological changes are not necessary for 

 the process of elaboration of the fluid. This assumption seems warranted also by 

 the fact that the embryonic fluid contains much more albuminous material than 

 does the adult fluid. 



The time of appearance of the chorioid plexuses in relation to the extraventric- 

 ular spread of the fluid would surely seem to offer undoubted evidence in regard to 

 the first elaboration of the fluid by the plexuses. It has been shown that in pig 

 embryos over 14 mm. in length the replaced solution in the cerebro-spinal system 

 spreads from the roof of the fourth ventricle into the periaxial tissues. This extra- 

 ventricular extension occurs practically simultaneously with the first indications, in 

 the pig embryo, of the formation of the chorioid plexuses of the fourth ventricle. 

 Thus, in a pig embryo of 14 mm., the primitive thickening and tufting of the epen- 

 dyma of the roof of the fourth ventricle may be observed (fig. 32). In earlier stages 

 no definite evidence of this developmental process is found. 



From the first indication of a developing chorioid plexus in a pig embryo of 

 14 mm., the growth of the tufts progresses rapidly, so that at 18 mm. the process is 

 well advanced. In embryos of 20 mm. and over the tufts of the plexuses in the 

 fourth ventricle are quite marked, as shown in figures 22, 44, 46, and 92. 



The chorioid plexuses of the third and lateral ventricles appear at a somewhat 

 later stage than do those in the more caudal ventricle. Thus the first indication of 

 their appearance in pig embryos is found in specimens measuring 19 mm. in length. 

 This coincides well with the further extension of the replaced fluid in specimens 

 of 19 mm. and over. The definite differentiation of these plexuses, however, does 

 not actually take place until the embryo reaches a length of 23 mm. a fact sug- 

 gestive of some relationship to the complete periaxial spread found in embryos of 

 this measurement. 



