Irving Harclesty '^iil 



Figs. 12 and 13). These scaJ-ving cells have definite boinidaries and 

 therefore cannot be confused with the neuroglia nuclei surrounded by a 

 mass of the S3'ncytial protoplasm. They usually encircle the medullat- 

 ing axone comjjletely^ though sometimes their protoplasm is so thin on 

 the side opposite the nucleus that they appear as a crescent rather tlian 

 a circle. They are not noticed till medullation has begun and they 

 appear more frequently about small and medium-sized azones than about 

 those Avhose medullary sheath has attained the larger proportions. They 

 seemingly increase in size with the thickening of the medullary sheath 

 they encircle. That shown in figure 13 is the largest I have observed 

 in any of my sections. They are more numerous during the period of 

 most active medullation (16 to 25 centimeters) and, so far, I have been 

 unable to note distinct examples of them in the adult material. If they 

 occur in the adult at all, their protoplasm must be either exceedingly 

 thin, or all used up, and their nuclei may be included among the appar- 

 ently " free neuroglia nuclei," some of which are often seen lying close 

 or even curved upon the periphery of the medullary sheaths. I have 

 never seen more th.an four of these cells in a single field of the oil immer- 

 sion. More often a field contains none at all. The cell shown in Fig. 

 13 {s) is entirely imique in the preparations. It apparently involves 

 two axones in its clasp. Whether this is due to ^^ressure or is indicative 

 of the nature of these cells, it is the only one observed l)ehaving in 

 this way. 



As to the function of these seal-ring cells, it is at present assumed 

 that they bear the same relation to the medullated axones of the central 

 nervous system as that ascribed to similar cells laiown to occur in the 

 peripheral nervous system. Adamkiewicz. 85, was, I think, the first to 

 fully describe such cells in the developing peripheral nerves. He re- 

 ferred to them as " nerve corpuscles " and " half-moon cells," and they 

 have since been called " Schwann's corpuscles " from their relation to 

 the sheath of Schwann and their supposed identity with the nucleus of 

 that sheath in the adult peripheral nerve. These cells are thought to be 

 actively concerned in the development of the myelin sheath. Consider- 

 ing the seal-ring cells in the developing central system as similar to the 

 nerve corpuscles in the peripheral nerves, it may be assumed that in the 

 central system also they have something to do with the development of 

 the medullary sheaths and, further, that they are likewise of mesodermal 

 origin, probably recruited from the wandering cells which are known to 

 enter the central system. Whether or not in either system these cells 

 play a role in the production of myelin similar to that played by the 

 fat-cell in the production of fat, it may at least be advanced that they 



