THE ECTOPLASM 



cell and its process, for example, of the sciatic nerve in man 

 which is made up of processes from nerve-cells in the spinal 

 cord, may be easily a meter in length. What we speak of as 

 a nerve is really a bundle of processes from nerve-cells. 

 When, for instance, one makes experiments in the labora- 

 tory on excised nerves — the sciatic nerve of the frog is a 

 favorite object for such — one uses only the ectoplasmic 

 portions of cells. Therefore, conduction by nerves means 

 conduction by ectoplasm. That the nerve-fibre is ecto- 

 plasm was fully demonstrated by Harrison's classic and 

 epoch-making work on the origin of the nerve-fibre. 



Harrison's researches while establishing the mode of 

 origin of the nerve-fibre also furnish a beautiful demonstra- 

 tion of ecto-endoplasmic differentiation. Harrison, the 

 originator of the now so widely used method of growing 

 tissues outside of the body of an animal, had as his object 

 the settlement of the much debated question of the mode of 

 origin of the vertebrate nerve-fibre from the central nervous 

 system. By growing young embryonic cells taken from the 

 spinal cord of a frog embryo at a stage in which he knew 

 that no nerve fibres had yet developed, he was able to prove 

 that these cells extend pseudopodial processes which by 

 farther growth become the nerve-fibre. Says Harrison^: 

 (Fig. 4-) 



From the time when the tissue is implanted in the lymph 

 it shows a tendency to spread out, and often broad laminae 

 made up of a single layer of cells are found at the peri- 

 phery of the mass, while individual cells may move off 

 entirely by themselves. This is the case with both nervous 

 and axial mesodermic tissue, as well as with pieces of ecto- 

 derm, though the latter more often roll themselves into com- 

 plete spheres. One notable peculiarity that has frequently 

 been observed is the formation of large round or oval open- 

 ings in the flattened tissue, which may be surrounded 

 by very narrow bands or rings of tissue with cells sometimes 

 in single file. This phenomenon may possibly be due 



^ Harrison, igio. 



S3 



