INJECTIONS AND REPLACEMENTS IN THE CEREBRO-SPINAL SYSTEM. 23 



In the pig embryo of 18 mm., shown in figure 4, the replaced solution has been 

 carried somewhat farther than in the embryo of 14 mm. (fig. 3). The chief point 

 of differentiation lies in the fact that in the latter stages two areas have apparently 

 become permeable to the intraventricular fluid, so that a larger periaxial spread has 

 resulted. Then, too, the extension of the ferrocyanide solution from the superior 

 area is considerably greater, overlapping the cerebellar lip and filling in some degree 

 the pericerebral tissue in the chorioidal imagination. 



With a definite periaxial spread established for the cerebro-spinal fluid in pig 

 embryos of 14 to 18 mm., it seemed not unreasonable to expect a gradual increase 

 in the extent of the future subarachnoid distribution in more advanced stages. The 

 earliest extension of the fluid into the peribulbar tissues occurred with the inception 

 of the infolding of the ventricular roof to form the chorioid plexuses of the fourth 

 ventricle. Its further extension, particularly its passages through a second area, 

 occurred with the greater development of the chorioidal invagination (i. e., 18 mm. 

 stage). A still more extensive pericerebral flow of the ferrocyanide and citrate is 

 illustrated in figure 5. Here the cerebro-spinal fluid in a living pig embryo of 19 

 mm. was replaced by the ferrocyanide solution. The embryo was kept alive for 

 about an hour after the replacement and was then fixed in toto in an acid fixing 

 medium, which caused the precipitation of the prussian-blue. On clearing subse- 

 quently by the Spalteholz method the spread of the solution was found to be some- 

 what more extensive than in the stage of 18 mm. (cf. figs. 4 and 5). In figure 5 the 

 whole periaxial area over the roof of the fourth ventricle is shown to be completely 

 filled by a dense aggregation of the prussian-blue granules. The separation of the 

 two areas of fluid passage can not be made out in such a specimen. This dense 

 periaxial extension almost completely covers the cerebellar lip, not only in the 

 medial region but laterally to the limit of the ventricular roof. The injection 

 precipitate lies directly beneath the skin in this area, but more posteriorly its 

 separation from the skin becomes more marked. Tracing this dense periaxial 

 injection posteriorly, it is seen (fig. 5) to end somewhat abruptly in the region of 

 the cephalic flexure. The line of termination of the denser mass, to the ventral 

 surface of the medulla, tapers somewhat anteriorly. This extraventricular spread 

 is medial to the otic vesicle, but extends peripherally along the caudal cerebral 

 nerves, reaching outward as far as the peripheral ganglia. The periaxial spread 

 also closely covers the ventral surface of the medulla and extends in this plane 

 around the pontine flexure for a short distance upwards along the basilar surface 

 of the mid-brain. 



Examined from its dorsal aspects, the superior portion of the spinal cord is 

 found to be covered (in a perispinal relation) by a fine deposit of the prussian-blue. 

 This is shown in figure 5. Caudally from the higher cervical region there is no evi- 

 dence indicating a further spread hi the perispinal tissues. Such a spread from 

 above downward is wholly at variance with Reford V 47 ) conception of a development 

 of the spinal meningeal spaces before the cerebral. The complete filling here of the 

 central canal of the spinal cord and of the cerebral ventricles with the replaced fluid, 



