GENERAL SUMMARY. 101 



XIV. GENERAL SUMMARY. 



In the foregoing sections of this communication some of the problems concerned 

 with the embryology of the cerebro-spinal spaces have been discussed and observa- 

 tions have been presented in the hope that a better conception of the processes might 

 obtain. It is purposed to present here briefly the results of these observations and 

 to attempt to correlate the findings so far as is possible; and in this, as in the detailed 

 reports in the preceding pages, the relationship of the physiological processes con- 

 cerned will be referred to the morphological changes in the developing embryo. 



As a means of studying the physiological extent of the embryonic cerebro-spinal 

 spaces, a method of replacing the medullary fluid with a foreign solution was devised. 

 The procedure consisted in substituting, in the living embryo, a solution of potas- 

 sium ferrocyanide and iron-ammonium citrate for the cerebro-spinal fluid. The 

 embryos were then kept alive, for periods of about an hour, by placing them with 

 the attached placentae in an incubator at 38. At the end of this time, which varied 

 in the many experiments, the whole embryo was fixed in a medium containing 

 hydrochloric acid, thereby precipitating an insoluble prussian-blue. Specimens 

 prepared in this manner were studied after sectioning or after clearing by the 

 Spalteholz method. 



Pig embryos, subjected to such experimental replacements, exhibited only an 

 intraventricular retention of the foreign solution until after a stage of 14 mm. was 

 attained. In the earliest specimens, embryos of about 9 mm., there was no charac- 

 teristic distribution of the foreign solution, except that it remained within the medul- 

 lary-canal system. In stages of about 13 mm. the replaced fluid also was retained 

 within the cerebral ventricles, but in these specimens a dense accumulation of the 

 precipitated prussian-blue may be made out in a distinct oval in the superior portion 

 of the rhombic roof. This granular aggregation occurs against a histological differ- 

 entiated area in the roof of the fourth ventricle an area which represents apparently 

 the more epithelial-like elements of the earlier roof-plate. This area must be consid- 

 ered solely as a differentiation of the epidermal lining of the medullary-canal system. 



In living pig embryos of 14 mm. and over, the result of the routine replacement 

 of the ventricular cerebro-spinal fluid was a slight extraventricular spread into the 

 tissues posterior to the rhombic roof. The passage of this foreign solution outward 

 occurred through the same area of ependymal differentiation, outlined by the col- 

 lection of granules against its inner surface in the previous stage. The extraven- 

 tricular spread remains definitely localized to a very small conical area which does 

 not rapidly increase in size. 



The factors which cause this initial flow into the pericerebral spaces are of 

 interest. It follows that in the growth of the embryo the production of the intra- 

 ventricular and intraspinal cerebro-spinal fluid must necessarily keep pace with the 

 increasing size of the cerebral ventricles. It is also necessary for the occurrence of 

 an extraventricular spread of the fluid that the production of the fluid within the 

 ventricles must exceed the amount required to keep the medullary-canal system 

 filled. From our knowledge of the elaboration of the adult cerebro-spinal fluid, it 



