46 DEVELOPMENT OP CEEEBRO-SPINAL SPACES IN PIG AND IN MAN. 



non does not occur to any appreciable extent in earlier stages or in other parts of 

 the mesenchyme, except about the nervous system. The close association of the 

 coagulum from the ventricular cerebro-spinal fluid with the inner border of the area 

 membranacea (shown in figure 75 as a slight roughening of the border) is of very 

 great significance in this connection. In one point in the membranous area (fig. 75) 

 the albumen can be traced almost without interruption from the ventricle into the 

 wide spaces of the mesenchyme (cf. fig. 8). This observation strongly suggests that 

 the embryonic cerebro-spinal fluid, which is rich in protein material, is passing, in 

 this stage of embryonic growth, from the ventricle into the periaxial mesenchyme; 

 and such an interpretation becomes established by the comparative findings in the 

 embryo of the same stage in which a replacement of the cerebro-spinal fluid by the 

 ferrocyanide solution had been effected. These comparable findings are surely of 

 the utmost importance for the final solution of the problems centering about the 

 embryonic cerebro-spinal fluid. 



In the later stages of development of the area membranacea inferior in the pig 

 embryo the same structural relationships persist that are shown in figure 75. 

 Figures 76 and 77 are photomicrographs taken from a sagittal section of a specimen 

 of 32 mm. In the enlargement of the squared area, from the first of these figures, 

 the continuity and completeness of the membrane are well established. The photo- 

 graph shows well the flattened character of the cells comprising the membrane and 

 its sharp differentiation from the nervous tissue and ependyma below and from the 

 ependyma and chorioid plexus above. Most important in this case is the distribu- 

 tion of the albuminous coagulum. Within the ventricular cavity this appears in 

 considerable amount, and in several places it is in close adhesion to the lining area 

 membranacea. This albuminous precipitate may likewise be traced in some places 

 apparently through the cellular membrane into the periaxial spaces. For here, as 

 indicated in figure 75, the clotted albumen from the cerebro-spinal fluid apparently 

 exists in large amounts in the space just posterior to the membrane the future 

 cisterna cerebello-medullaris. Delicate strands of mesenchyme are still observed 

 running through the wide space, but in general the whole tissue has returned to the 

 line of the future arachnoid. The relative lack of substantial support of the mem- 

 brane is well brought out in figure 77. A characteristic feature of this membrane, 

 which Blake (3) has championed, and which is indicated in figures 76 and 77, is the 

 posterior bulging of the roof "the caudal process like the finger of a glove." 



Another section from the same pig embryo, taken more laterally, is represented 

 in figures 78 and 79. In the photomicrograph of higher power the flattened char- 

 acter of the lining cells, the intactness of the membrane in isolating the ventricular 

 cavity, the unsupported freedom of the membrane, and the relation to the albumen 

 coagulum on both sides are of particular interest. 



The ultimate fate of the area membranacea inferior will not be more fully 

 entered into until the early history of the similar area in the human embryo has been 

 detailed. For in this connection the occurrence of the foramen of Magendie requires 

 discussion, and it seems best to delay the further consideration of the present topic 

 until the whole question can be reviewed. 



