THE PERINEURAL SPACES IN THE PIG EMBRYO. 99 



In the course of this investigation of the cerebro-spinal spaces interest naturally 

 turned to the perineural spaces. In the typical experiments (a replacement of the 

 embryonic cerebro-spinal fluid with a demonstrable true solution in the living 

 embryo), there was evidence of a spread of the replaced solution around the cranial 

 nerves. Because of the procedure used (merely a filling of the ventricles and central 

 canal of the spinal cord) no evidence of a perineural spread occurred until the foreign 

 solution passed into the periaxial tissues. Here the spread chiefly involved the 

 caudal cranial nerves curving around the lateral surface of the medulla in fan- 

 shaped processes (figs. 5, 6, 8, and 9). The spread, however, was not extensive. 

 In figure 8 a similar slight spread along the spinal nerves is to be made out. Closer 

 study of these cleared specimens, and examination of the same and of similarly 

 injected embryos after serial sectioning, convinces one that the apparent perineural 

 spread in these cases extends around the sensory ganglia and not further toward the 

 periphery. In no case, either in the caudal portion of the cranial or in the spinal 

 region, has the replaced injection fluid passed the blastemal condensation of mesen- 

 chyme. This finding is well shown by the distribution of the injection fluid in 

 figures 9, 16, and 18. 



The optic nerves, however, possessing ganglia in the retina, usually show, in the 

 typical replacements in the living embryo, a partial or complete surrounding of the 

 nerves by the precipitated prussian-blue. An incomplete example of this more 

 typical, according to these observations, than a total circumvention is given in 

 figures 19 and 20. The higher-power reproduction of this field is very interesting. 

 It shows in the central portion the fiber bundles comprising the optic nerve, sur- 

 rounded by mesenchyme and the developing ocular muscles. In the region between 

 the nerve and the muscles is an undifferentiated mesenchyme which is characterized 

 by a crescent of the precipitated granules of prussian-blue. The non-penetration of 

 the surrounding tissue by the ferrocyanide is very well brought out in this drawing. 

 The prussian-blue has reached its position about the nerve by extension from the 

 pericerebral spaces; actually it has still the same distribution as noted in figure 8 

 above. The adult dura will completely surround the optic nerve in its whole extent; 

 the subarachnoid space will likewise extend unbroken to the posterior surface of the 

 eyeball. Hence it must be assumed that in this case the perineural space does not 

 extend beyond the peripheral ganglion. With regard to the olfactory nerves, no 

 evidence of a perineural spread was obtained in specimens of pig embryos up to 45 

 mm. in length. 



It seems obvious, then, that in the embryo pig true solutions, when substituted 

 for the cerebro-spinal fluid, do not extend peripherally along the nerves any further 

 than does the dura in the adult. The replaced fluid (if, as appears most likely, it 

 indicates the true circulation of the cerebro-spinal fluid) extends only through the 

 future subarachnoid space. Such a conclusion is best supported by the observations. 

 The only discrepancy between the findings in the pig embryo and those in the adult 

 with the same method lies in the fact that in the adult the cranial nerves showed a 

 much more extensive perineural injection. This seeming discrepancy may be 



