2 GO The Development of the Neuroglia 



digested material. All the resisting tissues stain practically the same. 

 Either the neuroglia fibers are digested out or, in the process of digestion, 

 the very properties upon which their differential staining is based are 

 destroyed. Undifferentiated, the presence or absence of neuroglia fibers 

 in the section is hard to determine, because, with the best of care, the 

 various washings necessary in staining the sections of digested material 

 result in a further collapse of the finer structures and a, coherence or 

 washing aside of the individual fibers. Also, while at times individual 

 fibers may be seen which resemble neuroglia fibers, being undifferentiated, 

 it is uncertain whether they are neuroglia or fibrils of white filn'ous 

 tissue. Digested preparations are of such a nature that, though they 

 may look promising under low power, they are very disappointing when 

 examined with the oil immersion, and this is necessary in the study of 

 neuroglia. Nothing in my preparations positively suggests the digestion 

 of neuroglia. The partial detachment of the pia from the white sub- 

 stance seemed at first sight to indicate that the cortex or marginal veil 

 had dissolved out, but under high power the general collapsed condition 

 of the resisting structures at the periphery is such that a definite state- 

 ment to this effect is impossible. The separation of the pia from the 

 cord may be due entirely to the general swelling and loosening of the 

 pia produced in the digesting process. Of the section in general, how- 

 ever, one can but say that if all the resisting framework of the spinal 

 cord is developed from the ingrowths of mesodermal tissue solely, then 

 mesodermal tissue must contribute for its support to a hitherto unac- 

 credited extent. 



A study of the resistance to digestion of the framework of the medul- 

 lary sheath is of interest, but since these structures probably have noth- 

 ing whatever to do with the neuroglia, their behavior is of necessity 

 omitted in this paper. 



The Occurre^^ce of Nerve Corpuscles, or Seal-rikg Cells, in the 

 Central Nervous System. 



There is one feature, however, to be observed in my preparations of 

 the developing spinal cord to Avhich I wish to call brief attention, though 

 it also may not be concerned Avith the neuroglia. It is the occurrence 

 of cells encircling the medullating neuraxes of the central nervous 

 system. At present only a mere mention of these structures is possi- 

 ble. An attempt to give them a more detailed study will be made at 

 another time. 



These cells api^ear clasping the growing medullary sheath and resem- 

 ble a seal ring in shape, with their one nucleus in the thicker side (s. 



