246 The Development of the Neuroglia 



none. Fitting in among the other nuclei, it may be compressed in the 

 opposite plane to that of the section and may therefore, in section, appear 

 smaller than others (see Fig. 5). If the germinal cells occur extra- 

 ependymal (which in pigs they very rarely do), just as the more scat- 

 tered non-dividing nuclei there, they may have a varying amount of the 

 more compact protoplasm about them, or they may have none at all, and, 

 owing to the plane of section, they may appear large or small. After the 

 blood-vessels have grown in, some extra-ependymal mitoses may be of 

 mesodermal elements rather than of those in question. 



As before suggested, after the first entrance of blood capillaries, the 

 framework of the central nervous system is no longer solely of ecto- 

 dermal origin. In addition to what is contributed by the walls of the 

 blood-vessels, the pia mater itself, which begins to take form at an early 

 stage, sends numerous ingrowths into the spinal cord. In pigs of 20 

 millimeters these ingrowths may be discerned in sections, and at 30 milli- 

 meters they are more marked (i, Figs. 8 and 9), and are sometimes 

 accompanied by nuclei from the mesodermal tissue of the pia. In the 

 later stages, after the development of the fibers of the white fibrous 

 tissue which occurs long before neuroglia fibers are differentiated, the 

 mesodermal ingrowths are easily seen in section stained by Mallor/s 

 method for white fibrous tissue. By far the greater contribution of 

 mesodermal tissue, however, is brought into the central nervous system 

 by way of the blood-vessels. The capillaries, first entering in pigs from 

 9 to 10 millimeters, carry in this tissue both as composing their walls 

 and their contents. As they branch and ramify, their walls thicken and 

 send processes into the surrounding ectodermal tissue. These pro- 

 cesses, just as those from the pia direct, are accompanied by nuclei of 

 mesodermal origin. Further, leucocytes have been observed passing 

 through the walls of the capillaries to wander into the tissue without. 

 Thus, if with this point in view, the changes are carefully followed into 

 the later stages, one is convinced that nuclei from these two sources con- 

 stitute an appreciable quota of those present in a section of the spinal 

 cord. The mantle layer (Eandschleier), at first thin and almost free 

 from nuclei, gradually thickens and gains nuclei (Figs. 6 to 10), and up 

 to 80 millimeters, at least a majority of the nuclei situated in it, whether 

 in mitosis or not, may be considered as mesodermal nuclei acquired in 

 the above manner. 



Attention has recently been called to these mesoblastic constituents 

 of the central nervous system. In a joint paper, Capobianco and Frag- 

 nito, 98, noted the manner of their ingrowth, migration and distribution 

 among the ectodermal elements. Later Capobianco, 02, attributes to 



