376 THE WONDER OF LIFE 



development are relatively exceptional, and the individual's 

 start in life is usually the fertilized egg-cell. 



The fertilized ovum divides and redivides, and we may 

 see this going on in the frog's spawn in the ditch. In that 

 case a groove appears dividing the ovum into a right and 

 left cell ; then another, at right angles to the first, dividing 

 each of these into an anterior and a posterior half ; then a 

 third cleavage in a horizontal plane cuts the four cells across 

 the equator, forming an upper hemisphere with four 

 smaller, and a lower hemisphere with four larger cells. In 

 some cases the process of cleavage suggests the operation 

 of an invisible magical knife. 



For a time the process of division continues without 

 there being any growth, and a ball of cells (or blastomeres) 

 is formed which is still no larger than the original unseg- 

 mented ovum. But growth soon begins, and the cells 

 are arranged in germinal layers, or are variously localized 

 by processes of infolding and overlap. Sooner or later 

 the cells begin to show differentiation, some laying the 

 foundations of the nervous system, others of the muscular 

 system, others of the digestive system, and so on. And 

 besides differentiation there is the process of integration 

 — ^the unification and co-ordination of the developing 

 organism. In short, there is a process of embryonic 

 development — condensing into a few days or weeks the 

 achievements of ages of evolution. 



At a certain stage, differing greatly in the different types, 

 the egg is ' hatched ', and there emerges from the egg- 

 envelope — a young creature which is a dehghtful miniatuie 

 of the adult, as in the famihar case of a chick, or a larva 

 very different from the adult, as in the case of caterpillar 

 and tadpole. The embryo is the quiescent stage within 



