258 The Development of the Neuroglia 



than in the human or hog. Being void of axones, of course no " neuro- 

 glia cells " appear in it. Its nuclei are simply embedded in a formless, 

 fibrillated protoplasm. By the silver method, being on the periphery 

 of the spinal cord, it is almost invariably clogged by the deposit, and 

 being thin is usually passed unnoticed. By the differential neuroglia 

 stain it appears as a dense plexus of blue-staining neuroglia fibers and 

 shows in sharp contrast with the adjacent pia mater, which is colored a 

 light, reddish-brown. The plexus of neuroglia sends processes among 

 the axones along its inner border. These processes simply express its 

 continuity with the general syncytium of the entire specimen. 



Some of the steps in the processes of the development and transforma- 

 tion of the neuroglia fibers are, as I see them in the pig, similar to certain 

 of the processes described by Mall, 02, in the development of the white 

 fibrous tissue from the connective-tissue syncytium. Since neuroglia, 

 the chief fibrous-supporting tissue of the central nervous system, bears 

 a close resemblance to white fibrous tissue in certain of its framework 

 arrangements, it would not be surprising if a close comparison of the 

 two should reveal similar stages in their development. 



A summary of the entire processes of the development and transforma- 

 tion of the neuroglia is given in the last section of this paper. 



Experiments with Digestion. 



The results of the application of the digestion method to the embryonic 

 and adult spinal cord have been disappointing as to their contributing 

 to a knowledge of the growth and properties of the neuroglia. In the 

 early stages, before medullation and before neuroglia fibers make their 

 appearance, the results obtained by digestion are quite positive. Up to 

 6 centimeters the embryonic spinal cord is so friable that it was found 

 necessary to remove the segments intact in the vertebral canal and sub- 

 ject the whole to digestion. In this way one could be^sure the piece of 

 spinal cord was not lost in the manipulation. 



The syncytium of the embryonic spinal cord digests in common with 

 the connective-tissue syncytium. In all the first stages the spinal cord 

 digests out entirely, and, also, even at 3 centimeters, there is left scarcely 

 a vestige of the embryonic meninges. In pigs of 4 centimeters, however, 

 while the spinal cord digests totally, the anlage of the dura and pia 

 mater, though thin, begins to positively resist digestion. At this stage 

 the meninges begin to stain by Mallory's method for white fibrous tissue. 

 Thus it is seen that as stainable and indigestible membranes (white 

 fibrous tissue), the meninges are evident long before neuroglia fibers 

 make their appearance. 



