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



spinal cord and basilar structures, forming the initial indication of the pia mater. 

 This phenomenon is indicated somewhat in figure 40, a photomicrograph taken 

 from a human embryo (No. 836) of 4 mm., the earliest stage here illustrated. 



The next essential change in the great differentiation of the meninges concerns 

 a blastema! condensation of this same mesenchymal tissue to form ultimately the 

 bony covering of the central nervous system and a portion of the dura; but between 

 these two zones of differentiation the mesenchyme remains for a time almost unal- 

 tered. A portion of this tissue will go to form the arachnoid membrane and the 

 trabeculse which mark off the subarachnoid spaces. This process in the formation 

 of the arachnoid will be discussed here; the formation of the pia mater and dura 

 will be detailed in succeeding divisions of the paper. The differentiation will be 

 discussed as a general process, in regard to both human and pig embryos, for in no 

 respect has any essential difference between the two been observed. 



The general character of the periaxial mesenchyme may be commented upon 

 here. The tissue is of a very loose and typical structure, forming a syncytial net- 

 work of rather small mesh, but fragile. The nuclei of the cells are oval, with a 

 definite chromatin content; the cytoplasm is largely devoted to the maintenance 

 of long processes which connect with adjacent cells. Adhering to the cytoplasmic 

 processes are very tiny albuminous coagula, of such small amount as to be hardly 

 noticeable; also in the meshes of the mesenchyme very small quantities of this 

 albumen may be identified. These albuminous coagula undoubtedly represent the 

 protein of the tissue fluids in the undifferentiated stages. 



THE FORMATION OF THE ARACHNOIDEA. 



A general consideration of the problems here involved will surely shed light 

 on some of the various factors concerned. It must be noted that in its develop- 

 ment this membrane proceeds from an undifferentiated but small-meshed mesen- 

 chyme into the adult structure which contains the relatively large cerebro-spinal 

 channels. Then, too, the enlargement of the tissue meshes in certain places as 

 the future cisternse must be enormous. Besides this necessary dilatation of the 

 spaces in the periaxial mesenchyme, the outer portion of the tissue must separate 

 from the future dura and form the outer surface of the arachnoid membrane. Here 

 the process must be one of tissue condensation and proliferation. A similar agency 

 is involved in the growth of the mesothelial cells which cover the outer surface of 

 the arachnoid and also the inner subarachnoid spaces. 



The general process, then, in the formation of the arachnoid membrane con- 

 cerns a thinning and readjustment of the primitive mesenchyme in certain areas, 

 while in others the process is reversed, the membrane reaching the adult form 

 through proliferative and condensing phenomena. Such alterative processes must 

 naturally result from the application of certain mechanical or vital agents in the 

 growth of the embryo. Is the mere growth of the central nervous system sufficient 

 to furnish these alterative agents, or must we Likewise trace the corresponding devel- 

 opment of the bony coverings of the brain and spinal cord? Neither factor seems 



