i 4 6 AGE, GROWTH, AND DEATH 



Figure 1 1, p. 50, shows us more clearly the principle of 

 structure of a nerve cell, for there we have the central 

 body of the cell composed of protoplasm with its nu- 

 cleus in the middle and a small spot in the centre of 

 the nucleus, and the long branching processes running 

 out in all directions which can take up nerve impulses 

 from other similar or dissimilar cells, as the case may 

 be, and carry them to the central body. To carry the 

 message out there is typically but one process, which 

 is different in appearance from the other processes 

 which carry the impulses in. The latter are branch- 

 ing and are therefore called the tree-like or dendritic 

 processes. Here is a single process (Fig. 11, Ax) 

 like a long thread to carry the impulses away, and 

 which is called the axon of the nerve cell. In this 

 case the modification of the shape of the cell has 

 adapted it to the better performance of its functions. 

 Notice also in these cells the enormous increase in 

 the amount of protoplasm as compared with the nu- 

 cleus. In the young cell of the rabbit germ, of which 

 I showed you several illustrations a few moments ago, 

 we had very little protoplasm for each nucleus, but 

 here the protoplasm has many, many times the volume 

 of the nucleus, and this is a relatively old cell. 1 



Next let us look again at the figure of the striated 



1 The nerve fibres of vertebrates are usually each surrounded by a protective 

 covering of cells, making a sheath. Kolliker pointed out in 1886 that the 

 sheath cells are very small in young embryonic stages and that their size increases 

 with age, owing not to the growth of the nuclei, but to the growth of the cell 

 body, including the myelin, the special substance which characterises the dif- 

 ferentiation of these cells. See " Ilistologische Studien an Batrachier Larven," 

 Zeitschr. fur wiss. Zoologie, xliii. , p. I. 



