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



Likewise in birds Sterzi was able to differentiate only two meninges the dura 

 and the "secondary meninx." These membranes are quite similar to those of 

 reptiles. The "secondary meninx" has acquired three layers an outer endothelial 

 covering, a middle vascular layer, and an inner membrane closely adhering to the 

 cord. This is a distinct approach to the three meninges of mammals. An intra- 

 dural (subdural) space covered by endothelium can be easily made out. The 

 development of these avian meninges concerns a differentiation of the perimedullary 

 mesenchyme. 



The arachnoid, according to Sterzi, first appears as a definite membrane in 

 mammals (marsupials and placentals). In marsupials this arachnoid has become 

 well differentiated and the pia mater possesses denticulated and ventral ligaments. 

 A transformation of the extradural portion of the denticulated ligaments unites the 

 dura to the endorachis. In perissodactyla the differentiation of the three meninges 

 (particularly of the arachnoid) is incomplete. The arachnoid is separated from the 

 pia mater by a peculiar tissue which contains numerous lymphatic lakes, forming 

 the intra-arachnoid spaces. No intradural (subdural) space is apparent, due to 

 the approximation of dura and arachnoid. The subdural space is clothed by endo- 

 thelial cells; these can not be made out in the intra-arachnoid spaces. The dura 

 is surrounded by a fatty pad. 



According to Sterzi the augmentation of the intra-arachnoid (subarachnoid) 

 space is the distinguishing characteristic of the meninges of carnivora. This 

 increase takes place at the expense of the peridural space. 



As Sterzi developed the knowledge of the comparative anatomy of the lower 

 forms of the transition from the primitive meninx of the cyclostomes to the three 

 membranes of mammals the possible correlation of this analogy to the embryo- 

 logical development in mammals became apparent. He extended his observations 

 to human beings and to human fetuses. His findings will be detailed in the follow- 

 ing section. 



Farrar (16 \ in a short discussion of the development of the meninges of the chick, 

 finds in early stages three laminae about the spinal cord, "the middle one of which 

 alone still presents the primitive features of the mesoblastic-sheath." The inner 

 layer, close to the medullary tissue, is highly vascular; in the outer zone "the con- 

 nective-tissue elements are assuming elongated forms and crowding together with 

 long axes parallel, giving a very close mesh with long but extremely narrow spaces, 

 in contradistinction to the loose irregular reticulum of the pia-arachnoid." The 

 outer lamina becomes dura mater, while the inner two zones are considered together 

 as the embryonic pia-arachnoid. Farrar defines the pia-arachnoid as develop- 

 mentally a single membrane consisting of a loose reticulum, at the outer and inner 

 borders of which limiting membranes are formed. 



