INHERITANCE 113 



minute structure of the egg, and of the changes 

 which it undergoes during development. This is a 

 perfectly philosophical standpoint to adopt ; for the 

 egg, say of a hen, has the power of developing 

 into a chick without any further assistance from 

 the parent, provided only that certain conditions of 

 temperature and moisture are fulfilled : and it 

 follows that the problem of heredity is centred 

 in the egg, and that it is therefore reasonable to 

 hope that patient investigation of the egg-structure 

 may throw some light on the question. 



Weismann in the first instance lays special 

 stress on the fundamental difference between 

 the unicellular animals, or Protozoa, on the one 

 hand, and the Metazoa, or multicellular animals, 

 on the other. In Protozoa there is no distinction 

 between body cell and germ cell : the entire animal 

 is but one single cell. Reproduction is effected by 

 simple division of this cell, and every individual 

 peculiarity in the parent must therefore be trans- 

 mitted directly to the offspring. Heredity therefore 

 in Protozoa is no problem at all, but a simple and 

 direct consequence of the mode of reproduction. 

 In the Metazoa however the case is different : 

 here the adult animal consists not of a single cell, 

 but of many cells arranged variously so as to form 

 the epithelial, muscular, nervous, and other compo- 

 nent tissues of the animal. Of these cells certain 

 ones are early distinguished as genital cells, and to 

 these the power of reproduction, at any rate sexual 

 reproduction, is confined, and in them are centred 

 the hereditary tendencies of the whole organism. 



H 



