Irving Hardesty 



339 



fiber possess sheath cells and it will be noted that all three are fibers upon 

 which the processes of medullation are well advanced. In the group 

 indicated hy a arc three fibers not separated bv the teasing and there is 

 still present al)out them the syncytial protoplasm and some of the nuclei 

 belonging to it (5). One of the axoncs of this group as yet shows no 

 evidence of myelin, a condition which is quite frequent in pigs of this 

 age. The fibers h and c were selected as showing the next stages in the 

 acquirement of myelin. The medullated fiber with group a was included 

 as illustrating the corrugated or ruffled outline of the growing sheath, an 

 appearance frequently found and which suggests that it is an earlier 

 stage than either of the fibers indicated by c. l)eing a further develop- 



FiG. 5. Types of nerve fibers selected from teased preparations of spinal 

 cord of a pig of 21 centimeters, showing stages of medullation and nature of 

 seal-ring cells. Osmic acid and corrosive sublimate. Fuchsin. s = syncyt- 

 ium; /"?•=: framework of sheath washed out in teasing; 5C^ seal-ring cells. 

 X 550. 



ment of the smaller Idistcrcd or lieadcd form {h and c). The filler d is 

 perhaps in about the same stage, but it is doubtful whether either of the 

 nuclei adhering to it represents sheath cells. The fiber f, showing an 

 even contour of its sheath, is considered as illustrating the type of the 

 most advanced stage in the growth of the medullary sheath found in 

 pigs of 21 centimeters. This type is fairly numerous and often sheath 

 cells are found upon it. The myelin of this type stains more darkly than 

 that of the others, especially that of h and c, where it is much lighter and 

 corresponds with certain of the descriptions of so-called " eml)ryonic 

 myelin." 



In the late foetus and in the new-l)orn (suckling pig of two weeks) 

 the conditions more nearlv resemble those of the ailult. j\redullation 



