172 The Unity of the Organism 



ism for more than eleven months, that its mass increases 

 considerably, and its power of proliferation, after such long 

 period, is more active than at the beginning of its life m 

 vitro/' ' In one culture, fragments of the heart pulsated 

 after 104 days. The evidence is now conclusive that various 

 tissues of numerous animals are able to live and grow and 

 perform something of their characteristic activities for a 

 long time after being separated in small fragments from the 

 organism at various stages of its development. 



Another striking attribute proved for the cells of young 

 embryos is their mobility. Harrison dwells on the exten- 

 siveness and significance of this, and all the other investi- 

 gators are impressed with it. The wandering about and 

 the putting out of protoplasmic processes by cells of various 

 sorts, notably by connective tissue and nerve cells, are men- 

 tioned by all writers. Burrows' account of the behavior 

 of the growing nerve fibers is particularly full and well 

 illustrated, and many of the facts are so significant that I 

 quote at some length from his description : "Growth of 

 the nerve cells is evident by filaments of various sizes. . . . 

 The slender filaments are composed of a hyaline homogeneous 

 protoplasm, while in the coarser bundles the homogeneous 

 character is altered by the appearance of delicate, longi- 

 tudinal striations. The latter bundles break up into many 

 fine filamentous branches. . . . At the end of each of these 

 growing filaments and branches is the characteristic thick- 

 ened amoeboid swelling. . . . This is an oval or round swell- 

 ing of the filament from which protrude many actively mov- 

 ing delicate pseudopodia. The growth of a fiber consists 

 in the great prolongation and enlargement of one of these 

 pseudopodia with a gradual moving outward of the end 

 knob along the pseudopod. The growth may be so rapid 

 that the end knob may entirely disappear, to reappear far- 

 ther out along the new grown part. . . . During this time 

 (48 to 72 hours) they may . . . reach a length of from one 



