EXPLANATION OF PLATES. 



KEY FOR FIGURE-LEGENDS. 



ami, area membranacea inferior. dine, dura mater cerebri (inner surface, in pmc, pia mater cerebri. 



ams, area membranacea superior. approximation with arachnoid). piili, precipitated prussian-bluc. 



cbl, cranial blastema. epc, epithelial-like colls lining ventricle. psn, reduced silver nitrate. 



eem, cisterna cerebellc-medullaris. epe, ependyma. saa, subarachnoid spaces. 



chp, plexus chorioideus. 4i ventriculus quartus. str, sinus transversus. 



PLATE I. 



FIG. 1. Drawing of a pig embryo of 9 mm., into the spinal central canal of which an injection of 0.5 per cent solution 

 of potassium ferrocyanide and iron-ammonium citrate was made under very mild syringe-pressure. The 

 embryo was fixed in Carnoy's fluid to which 1 per cent hydrochloric acid had been added. The speci- 

 men was carefully dehydrated and cleared by the Spalteholz method. The resultant precipitate of 

 prussian-blue is found wholly within the central canal of the spinal cord and within the cerebral ventricles. 

 Enlargement, 11 diameters. 



FIG. 2. Drawing of a pig embryo of 13 mm., in which the cerebro-spinal fluid was replaced by a 1 per cent solution of 

 potassium ferrocyanide and iron-ammonium citrate. The embryo was kept alive for 90 minutes after 

 this replacement and was then fixed in 10 per cent formol containing 1 per cent hydrochloric acid. After 

 dehydration the specimen was cleared by the Spalteholz method. The occurrence of a definite oval, 

 outlined by the denser mass of the granules, in the roof of the fourth ventricle, is characteristic of this 

 stage. Enlargement, 9 diameters. 



FIG. 3. Drawing of a pig embryo of 14.5 mm. in which the cerebro-spinal fluid was likewise replaced by the ferrocya- 

 nide solution. After the replacement, the embryo was kept alive for 60 minutes; it was fixed in Carney's 

 fluid (with 1 per cent hydrochloric acid added) and after dehydration it was cleared by the Spalteholz 

 method. The earliest indications of a periaxial spread of the replaced fluid from the roof of the fourth 

 ventricle is here shown. Enlargement, 8 diameters. 



PLATE II. 



FIG. 4. Drawing of a pig embryo of 18 mm., in which a typical replacement of the spinal fluid had been made. The 

 animal was kept alive for 45 minutes and was then fixed, dehydrated, and cleared in the usual manner. 

 The extraventricular spread of the replaced fluid from two areas in the roof of the fourth ventricle is 

 well illustrated. Enlargement, 9 diameters. 



FIG. 5. Drawing of a pig embryo of 19 mm., in which likewise a typical replacement of the cerebro-spinal fluid by the 

 ferrocyanide solution had been made. After this procedure, the embryo was kept alive for 55 minutes 

 and was then carried through the routine technique for the Spalteholz method. The further pericerebral 

 spread of the replaced fluid is recorded. Enlargement, 8 diameters. 



PLATE III. 



FIG. 6. A frank lateral drawing of a pig embryo of 21 mm. The typical replacement of the embryonic cerebro-spinal 

 fluid by the ferrocyanide solution was effected in this embryo and it was then kept alive for 45 minutes. 

 At the end of this time the embryo was fixed in an acid fluid, dehydrated, and cleared. The almost 

 complete periaxial spread of the replaced fluid is indicated by the precipitated granules. Enlargement, 

 7.6 diameters. 



FIG. 7. A dorsal view of the embryo illustrated in fig. 6. The perispinal spread of the replaced fluid is well shown. 

 Enlargement, 7.8 diameters. 



PLATE IV. 



FIG. 8. Drawing of a pig embryo of 26 mm. in which the typical replacement of the cerebro-spinal fluid has been made. 

 After the introduction of the ferrocyanide solution the embryo was kept alive for one hour; at the end 

 of this time it was fixed in an acid solution, subsequently dehydrated, and cleared in oil of wintergreen. 

 The specimen shows a complete periaxial spread of the replaced fluid, as evidenced by the precipitated 

 granules, in addition to a total filling of the intramedullary system. Enlargement, 6.5 diameters. 



FIG. 9. Drawing of a pig embryo of 16 mm., in which the central canal of the spinal cord was injected with the ferro- 

 cyanide solution under moderate syringe-pressure. After fixation in an acid medium the embryo was 

 dehydrated and cleared by the Spalteholz method. The extraventricular snread in the peribulbar region 

 is easily made out. Enlargement, 9 diameters. 



Ill 



