248 The Development of the Neuroglia 



elastic fibrous tissue, some cartilage, etc. Within the central nervous 

 system and subjected to the environment there, some may contribute to 

 the formation of neuroglia. 



If the term " neuroglia " includes the entire framework supporting 

 the central nervous elements, then of course mesodermal elements con- 

 tribute to it. But, such of the supporting tissue as can be distinctly 

 designated as having mesodermal origin is of the white fibrous variety. 

 Sections of the adult spinal cord prepared by a method differential for 

 white fibrous tissue show abundant ramifications of this tissue. If, on 

 the other hand, the term " neuroglia " includes only that portion of the 

 framework which is differentiated by the special neuroglia stains, then 

 it becomes difficult to say, from my preparations of pig embryos, whether 

 any of the tissue is of mesodermal origin or not. Just as that portion of 

 the framework, which is of undoubted ectodermal origin, early assumes 

 and maintains the form of a syncytium, the same form of development 

 has been conclusively shown (Spuler, 96, Mall, 02) for the connective 

 tissues outside the central nervous system. And, as said before, the 

 mesodermal ingrowths begin before fibers are developed in either syncy- 

 tium, and the result is a fusion of the substances from the two sources 

 with no means of determining where the one begins or the other leaves 

 off. The nuclei of the two migrate and intermix and, with the exception 

 of nerve-cell nuclei and those of the ependymal layer, it becomes impos- 

 sible to tell the source of a nucleus by its appearance or position. All 

 these nuclei have been often referred to collectively as neuroglia nuclei. 

 While these so-called neuroglia nuclei begin to undergo variations long 

 before neuroglia fibers begin to appear, yet for some time after the varia- 

 tions begin to appear in my preparations, for any type of nucleus found 

 in the spinal cord a similar type may be distinguished in the embryonic 

 connective tissue outside. 



Neuroglia, to be distinguished as such, must possess those properties 

 which characterize it, but these characteristics do not appear till after 

 the intermixing of the material from the two germ layers. Morj)ho- 

 logically, neuroglia fibers are similar to those of white fibrous tissue in 

 some of its less compact arrangements. That neuroglia fibers differ in 

 their microchemical properties from those of white fibrous tissue is the 

 chief means by which the one may be distinguished from the other. By 

 the special neuroglia stains (those of Weigert and Benda) the easily dis- 

 cernible ingrowths from the pia certainly do not give the stain reaction 

 which characterizes neuroglia, and tests made of these methods upon 

 various tissues (Hardesty, 02), indicate that the methods give trustworthy 

 differentiation. On the other hand, should any of the fibers of white 



