GENERAL HISTOLOGICAL DIFFERENTIATION OF CEREBRO-SPINAL SPACES. 69 



greatly in size, there is still an intimate connection between dura and arachnoid. 

 These connections are particularly prominent over the developing cerebral hemi- 

 spheres, and it is with this differentiation in the formation of the arachnoid spaces 

 that we will now deal. 



In a human fetus of 76 mm. (No. 1134) the arachnoid was found to constitute, 

 in the region about the great sagittal sinus, a cellular layer which adhered quite 

 closely to the dura, even though a line of differentiation between the two meninges 

 could be made out. This adhesion could undoubtedly be separated, even by gross 

 dissection, although the tendency to adhesion was stronger than the attachment of 

 the pia to the cortex. From its cell-character and general histology the arachnoid 

 at this stage must be considered as a formed membrane, but in a primitive state. 



A somewhat similar but more advanced stage in the formation of the arachnoid 

 membrane is seen in a human fetus of 100 mm. (No. 928-E) and in a fetal pig of 

 114 mm. In both the arachnoid membrane is very cellular, adhering to the dura 

 only along the superior longitudinal sinus and in certain isolated areas. The cells 

 comprising the arachnoidea possess oval, rather large nuclei which stain palely with 

 hematoxylin. No typical arachnoidal trabecuke could be made out in specimens in 

 this cortical region. 



The cellular character of the arachnoid persists in the larger embryos and fetuses 

 as a layer, several cells in thickness, constituting the outer arachnoid membrane. 

 In a fetal pig 190 mm. in length the membrane was practically differentiated, its 

 outer wall being covered by mesothelial cells with large nuclei lying about a small 

 fibrous-tissue base. The arachnoid trabeculaB were developed only in the larger 

 sulci, where they appeared as typical cellular cords about a core of fibrous tissue. 

 At this stage, too, the vessels traversing the arachnoid spaces were found covered 

 with similar cells. These may now be justly termed the mesothelial cells. 



Quite similar stages of arachnoidal differentiation occur in human fetuses of 

 200 (No. 870) and of 240 mm. (No. 1131). The arachnoid has everywhere prac- 

 tically become adult in character, except for a further decrease in the number of the 

 peripheral layers of mesothelial cells. The fibrous tissue underlying this covering 

 membrane possesses, as in the adult, almost a minimum of support. 



In certain areas, however, the differentiation of the mesenchyme into the adult 

 arachnoidea does not keep pace with the general process. In the present study this 

 phenomenon of unequal development was especially well shown in fetal pigs of 

 150 mm. and upwards. It concerns the development of arachnoid trabecute in the 

 cerebral sulci. As is well known, the arachnoid membrane bridges the cerebral 

 fissures, while the pia follows the cerebral contour. In the fetal pigs of the stages 

 specified above, certain furrows showed a typical adult relationship with the covering 

 arachnoid membrane and lining pia, the intervening space being traversed by defi- 

 nite arachnoid trabeculse. Other of the sulci were filled with an almost embryonic 

 type of mesenchyme a loose meshwork of cytoplasmic processes containing rather 

 small oval nuclei. The explanation of this embryonic type of tissue seems to be 

 that it occurs in the newly developing sulci and that some time must elapse in this 



