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



impression that the subdural space is in many respects a serous cavity will probably 

 finally have greatest support ; intimate connections with the lymphatic system are, 

 however, entirely lacking in the dura. 



The development of the subdural space must necessarily follow the develop- 

 ment of the dura. It has been mentioned that in fetal pigs of 50 mm. the dura can 

 be freed from the arachnoid by gross dissection, but that at this stage many areas of 

 adhesion between the two membranes exist. Such an observation has considerable 

 bearing on the subdural space. For in the development of this space two processes 

 must proceed far enough to permit the separation of the dura and arachnoidea by 

 the capillary layer of fluid. The first of these processes, in order of probable impor- 

 tance, concerns the condensation of mesenchymal cells to form the outer membrane 

 of the arachnoidea. This thickening and resolution into a true membrane takes 

 place in close approximation to the inner surface of the dura. The second factor 

 concerns the covering of this inner surface of the dura with mesothelial cells. 



The lining of the subdural space by mesothelial cells can be readily demon- 

 strated on the inner surface of the dura by silver reductions, but the outer membrane 

 of the arachnoid does not permit of a similar technique. This technical failure in 

 regard to the outer arachnoid surface is probably to be accounted for by the dis- 

 similarity in cell-structure in the two situations. Similar difficulties have been 

 encountered by other observers. 



In order, then, to ascertain, if possible, at what stage a really adult subdural 

 space could be demonstrated, the inner surface of the dura from fetal pigs of various 

 lengths was subjected to treatment with silver nitrate. After the reduction had 

 taken place to a sufficient degree, the whole dura was washed with distilled water, 

 stained with hematoxylin, and cleared in glycerin. The pictures afforded by this 

 method were quite satisfactory, and the technical procedure was so simple and 

 reliable that considerable faith could be placed in the absence of the intercellular 

 reduction lines. 



The smallest fetal pig in which a typical mesothelial cell-pattern could be dem- 

 onstrated on the inner surface of the cranial dura was one of 50 mm. In this speci- 

 men the inner surface of the dura was not uniformly covered with the mesothelial 

 cells; certain ragged areas seemed to represent the points of adhesion of the arach- 

 noid to the dura. Figure 108 is a reproduction of a drawing made from one of the 

 areas in this specimen where a mesothelial cell pattern could be seen. The drawing 

 shows many of the characteristics of mesothelial cell-patterns of other parts of the 

 body. The irregularities in the cell-borders, the frequent accumulations of the 

 reduced silver in the cellular angles, and the general cellular pattern are quite typical; 

 but the variation in the size of the cells, as shown in figure 108, is also somewhat 

 different from the usual finding in the adult, where there is considerable constancy in 

 the size of the cells. About half the cells in this fetal pig of 50 mm. are diminutive in 

 size; the smallest are hardly a third the size of the largest. Transitions between 

 the smallest and largest cells are also shown in this figure. It is difficult to ascertain 

 whether these smaller cells represent young elements which have not yet reached 



