INJECTIONS AND REPLACEMENTS ]N THE CEREBRO-Sl'INAL SYSTEM. 21 



roof. This oval area is comparatively large and comprises a portion of the superior 

 or anterior half of the ventricular roof. This area, differentiated from the remainder 

 of the rhombencephalic roof, is clearly shown in figure 2, a drawing of a cleared speci- 

 men of this stage. 



With the exception of this strikingly dense area in the rhombic roof, the injec- 

 tion spread in an embryo of 13 mm., subjected to replacement of the cerebro-spinal 

 fluid by the fcrrocyanide, differs in no way from that in the embryo of 9 mm. Care- 

 ful inspection of figure 2 is convincing that the spread still remains within the 

 medullary canals, with no extension of the fluid into the spaces outside of the cerebro- 

 spinal axis. It seems justifiable, then, to speak of the cerebro-spinal spaces at this 

 stage of development as being only intramedullary in type, with no indication as 

 yet of a meningeal fluid cushion (corresponding to the adult subarachnoid space). 



With the use of larger embryos, however, for the medullary replacement with 

 ferrocyanide and citrate, the picture gradually changes. The first indication of a 

 more advanced stage of development is obtained in embryos whose length exceeds 

 14 mm. Figure 3, of a pig embryo of 14.5 mm., is included here as representing 

 this further extension of the injection fluid. The cerebro-spinal fluid of this speci- 

 men was replaced, by the compensating mechanism, by a solution of potassium 

 ferrocyanide and iron-ammonium citrate. The embryo was then kept alive (as 

 judged by the heart-beat) for a period of one hour. At the end of this time it was 

 fixed in an acid medium and subsequently cleared in oil of wintergreen after careful 

 dehydration. 



The essential differences between an embryo of this stage and one of the stage 

 represented in figure 2 concerns the spread of the injection fluid from the roof of 

 the fourth ventricle. Both specimens show a complete filling of the intramedullary 

 system (cerebral ventricles and central canal of the spinal cord) with the precipitated 

 prussian-blue granules. The specimen of 13 mm. (fig. 2) is characterized by a 

 dense oval collection of the prussian-blue on the upper and inner surface of the 

 rhombic roof. In the specimen shown in figure 3, in contradistinction to this 

 localized aggregation of granular matter, there is a delicate extension of the injec- 

 tion fluid caudalwards from the roof of the fourth ventricle. This fusiform projec- 

 tion is here readily made out, lying beneath the skin over the ventricular roof and 

 separated quite distinctly from the easily discernible line of the roof. This outward 

 extension of the fluid has a fairly wide and deep origin from the upper portion of 

 the roof, but tapers caudally to a sharp point with considerable rapidity. 



At the stage of 14 mm. the roof of the fourth ventricle shows the small depres- 

 sion which marks the formation of the chorioid plexuses. With this depression 

 occurring transversely the relation of the external surface of the embryo to the 

 ventricular roof necessarily alters somewhat in this region. The chorioidal depres- 

 sion of the roof gradually becomes separated from the skin; and it is into this area 

 between the skin and the ventricular ependyma that the first spread from the 

 cerebral ventricles occurs. At this stage, illustrated in figure 3, the injection is 

 intramedullary in type, with but slight extension into the pericerebral tissues. 



