THE DEVELOPMENT OF THE CRANIAL DURA MATER. 87 



their maximal growth; no evidence of cellular division, as evidenced by mi to tic 

 figures, has been observed, although in this connection it must be granted that the 

 cleared specimens are hardly the most favorable. Undoubtedly this explanation of 

 the smaller cells would seem to be the true one, but there is little proof for the view, 

 except their absence from the adult dura and their disappearance in larger specimens. 



This disappearance of the smaller mesothelial cells is not rapid, but is seemingly 

 delayed over into the larger fetuses; thus, in figure 109, a similar preparation from a 

 fetal pig of 75 mm., corresponding smaller cells are outlined. On account of the 

 absence from the field of the drawing of the larger elements, these cells do not appear 

 relatively as great in number as in the preceding figure. Likewise, in figure 110 

 every gradation in cell-size is shown, in a specimen made in the same manner from a 

 dura of a fetal pig of 90 mm. 



Very slowly in the course of growth of the fetus the cells lining the inner surface 

 of the dura reach their standard size and compose the mesothelial surface, with 

 very little variation in size. The process, however, is apparently very tardy, even 

 though the fetus at 16 cm. shows an inner surface to the dura which is largely com- 

 posed of standard cells (fig. Ill); but even in this figure, from a relatively large 

 fetus, the standard size of the cells has not been attained, for a few cells of small 

 size appear in the drawing. In other respects the whole pattern, in general appear- 

 ance, resembles closely the adult. 



It seems most fair to assume that the occurrence of a true mesothelial cell- 

 pattern on the inner surface of the dura represents the initial establishment of a 

 subdural space. On this basis the subdural space may be said to occur in fetal 

 pigs 50 mm. in length; in the present investigation it has been found impossible 

 to demonstrate the existence of the mesothelial cell-pattern in fetuses smaller than 

 50 mm. The separation of the dura, possible by gross dissection in pig fetuses of 

 40 mm., suggests that the space may be found at a slightly earlier stage than that 

 in which the mesothelial cells have been demonstrated. 



Anatomically the subdural space in pig fetuses resembles in every particular 

 the adult space in cats and dogs; this was described in a paper^ 55 ) published in 1914. 

 In the large pig fetuses injections of solutions of potassium ferrocyanide and iron- 

 ammonium citrate were made into the spinal subarachnoid space. After precipi- 

 tating the foreign salts as prussian-blue, the injection is found to be wholly within 

 the subarachnoid spaces, both in the spinal and cranial regions; the subdural space 

 is absolutely free from any evidence of connection with the subarachnoid space. 

 These findings wholly accord with the opinion concerning the adult subdural space 

 which has been repeatedly expressed. 



THE COMPETENCY OF THE EARLY DURA AS A CELLULAR MEMBRANE. 



During the stage when the condensation of mesenchyme to form the cranial 

 blastema is pronounced the spread of the cerebro-spinal fluid becomes more and 

 more extensive. In these stages, when the pig embryo measures from 16 to 25 mm, 

 approximately, the outer membrane of the arachnoid is not yet formed, the arach- 



