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



is impossible to conceive of the production of a true cerebro-spinal fluid in the peri- 

 medullary mesenchyme. Such a view would be a reversion to the old hypothesis 

 of Haller, who regarded the leptomeninges as the elaborators of the fluid. Likewise, 

 the passage of the replaced foreign solution into the extraventricular spaces would 

 render such a hypothesis untenable. 



Hence, it becomes incumbent to regard such an extraventricular spread of the 

 experimental solution as an indication that the production of the cerebro-spinal 

 fluid within the cerebral ventricles exceeds the capacity of the ventricles to care for 

 the fluid. This argues strongly that the process of elaboration of the fluid in these 

 pig embryos of 14 mm. is no longer sluggish, but that an active production, sufficient 

 to cause a slight extraventricular flow during the observation, is now taking place. 

 This acceleration of the flow is not great, but it represents a marked change hi 

 the relationship of the process of fluid elaboration to the increasing volume of the 

 ventricles. 



It seemed desirable to endeavor to correlate this extraventricular spread of 

 the experimental fluid with the morphology of some intraventricular structure at 

 this critical stage of 14 mm. in the pig embryo. The first evidences of villous 

 tufting in the chorioid plexus of the fourth ventricles were found to occur at this 

 stage in the pig. Other studies of this plexus, particularly those which concerned 

 the occurrence of glycogen in these glandular cells, were found to offer no additional 

 evidence of value in regard to the onset of function hi these structures. The corre- 

 spondence between the initial tufting of the ependyma to form the rhombic chorioid 

 plexuses and the initial extraventricular spread must be regarded as of the utmost 

 importance. It would appear most likely that as soon as the chorioid tufts occurred 

 an increased production of cerebro-spinal fluid took place, necessitating an extra- 

 ventricular expulsion of the excess of fluid. Such a view receives the utmost support 

 from these recorded observations; it is in keeping with the best conceptions of the 

 processes of production of cerebro-spinal fluid in adult mammals. 



With the initial pericerebral extension of the experimental fluid occurring hi 

 pig embryos of about 14 mm., the further extension of this spread did not occur until 

 after a length of 18 mm. was attained. At this stage the replaced foreign solution 

 passed from the fourth ventricle through two areas in the roof-plate. The chorioid 

 plexuses now have divided the roof into two portions; from each, fluid escaped. 

 The superior area of fluid passage is the same which was concerned in the initial 

 outpouring of the ventricular fluid. The inferior area, like the superior, is an area 

 of ependymal differentiation, of which the first evidence may be made out in pig 

 and human embryos of 15 mm. This differentiation consists in the transformation 

 of the densely staining ependymal elements into cells with larger nuclei, poor hi 

 chromatin, and with more abundant cytoplasm. 



After the functional employment of the two membranous areas is established at 

 about 18 mm. hi the pig, the further pericerebral spread of the replaced solution 

 occurs very rapidly. The peribulbar tissues are filled with the fluid and from this 

 region extensions occur downward into perispinal spaces and upward into the more 



