116 CARNEGIE INSTITUTION OF WASHINGTON. 



perispinal spaces occurs. Simultaneously with this caudal enlarge- 

 ment of the fluid-spaces the fluid may be traced along the ventral 

 surface of the mesencephalon. In pig embryos of 23 imn. these replace- 

 ments show a total filUng of the ventricular system with the blue and 

 an almost complete surrounding portion of the cerebro-spinal axis 

 with the granules. The superior portion of the mesencephalon and 

 the cerebral hemispheres are the last parts of the nerv^ous system to 

 show evidence of a pericerebral investment. This final estabUshment 

 of the adult cerebro-spinal relationship occurs in pig embryos of 

 about 26 mm. 



The rapidity of the spread of the fluid in embryos between 18 and 

 23 mm. is apparently due to the marked acceleration in the rate of 

 production of the ventricular cerebro-spinal fluid, which is coincident 

 with the fonnation of the chorioid plexuses of the third and fourth 

 ventricles. As soon as these tufts develop, the fluid is produced in 

 amounts that far exceed the quantities for which the more slowly 

 enlarging ventricles can provide. 



At no time was there evidence of the passage of cerebro-spinal fluid 

 from the ventricular system into the pericerebral and perispinal space 

 through any place except the two localized areas in the two portions of 

 the rhombic roof. These two opendymal areas both represent intact 

 membranes, and are apparently differentiated for the passage outward 

 of the ventricular fluid. Both are differentiated at a sUghtly earlier 

 stage than that at which they function actively. In the smaller 

 embryos the superior area membranacea fulfills by far the more promi- 

 nent funtion, but after the establishment of the lower point of fluid- 

 passage it undergoes regressive changes. The area membranacea 

 inferior continues to develop and function actively. It gradually 

 forms, with increasing growth of the embryo, a caudal projection 

 beneath the cerebellum. This inferior area in the roof gradually occu- 

 pies with its differentiated ependynia the whole velum chorioideum 

 inferius, and remains as a functional membrane during early fetal life 

 and possibly throughout the adult existence. 



Along with the investigation of the spread of the cerebro-spinal fluid, 

 a study was made of the character of the tissues through which the 

 spread occurred, and a full description of the development of the 

 cerebro-spinal spaces in the pig and in man is given in Dr. Weed's 

 papers. He describes the subarachnoid spaces as developing out of the 

 periaxial mesenchyme. This process involves the transfonnation of 

 the small "tissue-spaces" of the mesenchyme into larger subarachnoid 

 channels. 



The first signs of the differentiation are seen in the basal portion of 

 the skull, in the region around the medulla oblongata, and consists 

 of a gradual increase in the size of the mesh. At first the process 

 appears as a mere spreading apart of the cell-bodies on the introduction 



