CHAPTER IV 

 MULTICELLULAR ORGANIZATION: ONTOGENETIC 



THE developmental history of a multicellular animal from the egg to 

 full maturity is known as its ontogeny. The soma or body of such an 

 animal is thus an aggregate of cells descended by successive divisions 

 from a single cell, the fertilized ovum or oosperm. These cells form 

 connected masses, resulting in one or more bodies or individuals, and 

 in such an individual it will be noticed that two things have occurred. 



First. The cells, during their successive generations, have grown 

 to be of several different kinds, each of which is adapted to perform 

 some particular function that the individual may be called to maintain, 

 and, 



Second. These different kinds of cells have been grouped apart or 

 together to form regions and relations with each other that will best 

 permit them to perform their peculiar functions. Such regions or asso- 

 ciations of cells, together with their products, are called tissues; or, 

 where certain tissues are very especially arranged apart from other tis- 

 sues, the tissue aggregate is known as an organ. 



These two conditions are known as differentiation and organization 

 respectively, and the ability of the creature to maintain its life and place 

 in the world depends upon the degree of efficiency which differentiation 

 and organization have attained in relation to the conditions under which 

 the creature exists. 



One of the most important differentiations occurs early in the life 

 history and represents the separation of all the cells that are to undergo 

 further differentiation from one or more of them that do not differentiate 

 fundamentally but remain as a store of the original material, to be used 

 later in building up other organisms of the same kind (see Chapter 

 XXI). These latter are called the reproductive cells (Fig. 17). They 

 undergo a very special differentiation of their own at their time of 

 maturity. They may be many or few, and some of them sometimes 

 appear to not only retain their original reproductive powers, but to also 

 perform, in a degree, some of the body functions. These may be con- 

 sidered, therefore, under such circumstances, as somatic cells. In this 

 case we have a slight differentiation as compared with the higher forms 



