150 INDUCTION AND ORGANISATION 



for the particular functions which each of them will have to 

 fulfil in the whole of the organism. The period of organ- 

 formation, therefore, is followed immediately by a phase in 

 which the differentiation of the tissues, or histogenesis, 

 dominates the picture. During this period are formed the tissues 

 of which the embryo consists, such as muscle tissue, connective 

 tissue, nervous tissue, cartilage, etc. It is a direct consequence 

 of the preceding chemodif f erentiation ; it is the visible ex- 

 pression of the diversity of the cells which, in an invisible 

 form, was already present. 



Histogenesis is the last important step in the realisation of 

 the structural plan. The egg has now given rise to an organized 

 whole, a complex of histologically different organs and tissues 

 with fixed topographical relations. In a word, an embryo has 

 been formed. 



