72 DEVELOPMENT OP CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



THE GENERAL HISTOLOGY OF THE PIA MATER. 



The findings in this investigation are wholly in accord with the conclusions of 

 His^ 25 ), of Kolliker^ 31 ), and of Farrar^ 16 ), that the pia mater is derived from the 

 middle germ-layer. In the earliest stages the mesenchymal elements may be made 

 out adhering to the outer portion of the primitive nervous system. In the course 

 of growth these cells are grouped about the mantle zone of the spinal cord in a rather 

 dense layer, two or possibly three cells in thickness, with the typical oval nuclei of 

 the mesenchymal elements. Certain stages of this process may be made out in the 

 figures in this paper. Thus, in a human embryo of 4 mm. (No. 836 of the Carnegie 

 collection) the mesenchymal elements form a definite layer around the neural axis 

 (fig. 41). The nuclei are oval in shape, possessing a moderate amount of chromatin, 

 and are found in a layer two cells in thickness. This membrane, with its fairly 

 scant cytoplasm, is sharply differentiated by its existence between two layers, in one 

 of which nuclei are wanting, and in the other somewhat widely separated the 

 mantle zone of the spinal cord and the periaxial mesenchyme. 



This typical arrangement of the mesenchymal elements about the cerebro-spinal 

 axis holds in almost unchanged form throughout the whole embryonic growth. 

 Thus, about the nervous tissue in figures 48 and 52 (from human embryos of 7 and 

 9 mm., respectively) the same condensation of the mesenchymal elements to form 

 the pia mater are made out. This appearance is so familiar that further description 

 in the later stages seems needless, but certain characters of this embryonic arrange- 

 ment seem to require comment. 



The general appearance of the pial layer is greatly altered by the early forma- 

 tion of the capillary blood plexus about the nervous system. This plexus tends to 

 render the pial tissue more cellular, on first microscopic examination, as the endo- 

 thelial channels branch greatly outside of the nervous tissue in this mesenchymal pia. 

 The general character of the pial layer, however, as a membrane with prominent 

 nuclei and scanty protoplasm, is not altered at all by the vascular plexuses. 



The ultimate fate of these undiff erentiated mesenchymal elements forming this 

 initial pial condensation is a gradual transformation of the cells into very low 

 cuboidal mesothelial elements constituting the adult pia. The transformation con- 

 cerns not only the differentiation of the cells but also a rearrangement so that the 

 original layer of two or more cells in thickness becomes finally of but a single cell in 

 thickness. The process, in a way similar to the development of the subarachnoid 

 spaces, begins in the basilar portions and spreads upward; the process, hence, may 

 often be studied in a single suitable specimen. 



More important, for our consideration, is the peculiar relationship of the pia 

 mater to the roof of the fourth ventricle, and in particular to the two areae mem- 

 branacese. In this situation, in place of the slight mesenchymal condensation which 

 characterizes the pia, and which Minot^ 40 ) pictures in his figure 114, the mesenchyme 

 seems altered. The condensation to form the pia, which takes place in other situ- 

 ations about the true nervous tissue, has not here occurred. This absence of the 

 typical pial arrangement may be noted even in very small embryos those in which 



