24 DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



with no evidence of a periaxial spread except in the region of the fourth ventricle, 

 indicates that in the pig embryo the adult human relationship between the cerebral 

 ventricles and the subarachnoid spaces endures. There is apparently in this 

 embryo no evidence of the foramina of Bichat and of Mierzejewsky, a finding in 

 accord with the observations of Dandy and Blackfan' 10 '. 



In the slightly larger embryos the further extension of the embryonic extraven- 

 tricular spaces progresses rapidly. Figure 6 represents such an extension in a pig 

 embryo of 21 mm., in which the normal cerebro-spinal fluid was replaced by a dilute 

 solution of potassium ferrocyanide and iron-ammonium citrate. In this specimen the 

 central canal of the spinal cord and the cerebral ventricles are completely filled with 

 the precipitated prussian-blue. But in addition there is almost a total filling of 

 the periaxial spaces. Viewed laterally the densest aggregation of the blue granules 

 is again in the region of the roof of the fourth ventricle. As in the embryo of 19 mm. 

 (fig. 5) , the whole extraventricular tissue posterior to this ventricular roof is filled 

 with the granules precipitated from the foreign solution. The spread from this 

 region is similar to that in the previous specimen, except in its far greater extent. 

 The granules may be traced caudalwards in the perispinal spaces to the point of 

 injection. The arrangement of the precipitated material, both within the central 

 canal of the spinal cord and surrounding it in the perispinal relationship, is well 

 shown in figure 7, a frank dorsal view of the same specimen represented in figure 6. 

 The greater density of the perispinal granules in the upper region of the cord, as 

 contrasted with the granules in the thoracic region, is probably of importance in 

 indicating the direction of the flow from above downwards. The increased amount 

 of the injection fluid in the region about the point of insertion of the spinal needle is 

 in all likelihood due to a local spread from the needle, such as frequently occurs in 

 a very limited area. The phenomenon may, however, be due to an actual increase 

 in the size of the potential perispinal space, though observations upon other embryos 

 of the same stage of development argue against this view. The segmental outlining 

 of the caudal portion of the perispinal space is to be noted in this figure. 



The cephalic regions in the specimen of 21 mm. show a quite extensive spread 

 (fig. 6), and there is the same general distribution of the granules about the medulla, 

 as in the specimen shown in figure 5. The rhombencephalon is completely sur- 

 rounded by the blue, the ventral sheet inclosing it tightly. Laterally the prussian- 

 blue is shown in a dense mass, in ultimate relation to the cranial nerves as they join 

 the brain-stem. The cerebellum is practically completely covered by the precipi- 

 tate; from the ventral portion of the pericerebellar granules the replaced solution 

 (as evidenced by the granules of prussian-blue) spreads forward and surrounds a 

 portion of the mid-brain. Only the ventral surface of the posterior half of the 

 mid-brain is circumscribed by the granules; anteriorly it is wholly surrounded by 

 the periaxial injection; more anteriorly the extension is limited to the mesial struc- 

 tures, leaving unsurrounded the cerebral hemispheres, although creeping between 

 the hemispheres and the mid-brain. 



The peculiar avoidance by the replacement fluid of the extreme dorsal half 

 of the mid-brain is also to be made out in the dorsal view of the specimen (fig. 7). 



