2 INTRODUCTION 



egg into cells. This so-called cleavage results in the formation 

 of a small lump of more or less uniform cells, the blast omeres. 

 A cavity develops in the centre of this cell lump, and the 

 vesicular germ is then called a blastula. In the period following 

 cleavage the germ goes through important changes, consisting 

 of a series of movements of cell groups. The blastomeres are 

 thereby arranged in several layers which in their turn will 

 divide into more or less well defined masses of cells. Together 

 these processes may be called the topogenesis of the embryo; 

 they serve to divide the germ into a number of organ primordia, 

 each of which contains the material for one definite organ or 

 group of organs of the embryo. At first, these primordia consist 

 of cells which are all very similar, and of an almost undifferen- 

 tiated, embryonic character. This changes during the next phase 

 of development, when each cell of the primordia specializes in 

 a particular direction; in this way the various tissues of the 

 body are formed. This is called the period of tissue differentia- 

 tion (histogenesis) . Once development has proceeded so far, the 

 organs and tissues of the embryo begin to take up their re- 

 spective functions. Development has by no means ended yet, 

 but in the subsequent period the functions of the organs play 

 a decisive role in their differentiation. This is therefore called 

 the functional stage of development. Finally, two categories of 

 change which occur at a very late stage may be mentioned, viz. 

 those which lead to full maturity of the organism, and those 

 of senility. The latter are mainly of a disintegrative character, 

 and eventually result in the death of the individual. A further 

 category that must be specially mentioned is formed by de- 

 velopmental processes that do not manifest themselves during 

 the normal, undisturbed life of the individual, but may lead to a 

 more or less complete regeneration, a restitution of the struc- 

 ture of the organism when this has been damaged. 



When we survey the outline of the course of development 

 just given, its most striking characteristic appears to be an 

 increase in spatial multiplicity. This is seen most clearly if we 

 compare the culminating point of development, the adult in- 

 dividual, with its starting point, the fertilised egg. On the one 

 hand we have a very complicated whole of organs, tissues and 



