378 THE WONDER OF LIFE 



ment, e.g. the changes associated with sexual maturity. 

 The fact is that, in studying development, we are considering 

 the living creature in its time-relations, and definition 

 is a matter of convenience. 



The first wonder of development is the minuteness of the 

 starting-point. Even when we use the comfortable word 

 potentiality, we find it difficult to deny the wonder of con- 

 densing a complex inheritance into a microscopic germ-cell. 

 An ovum the size of a pin's head is a large ovum, as ova go. 

 Many are microscopic, and a spermatozoon may be only 

 forooo^h f the ovum's size. Can there be room in these 

 minute vehicles for the complexity of organization which 

 an inheritance implies ? 



The wonder grows when we consider some of the facts 

 of modern embryological research. Prof. Delage cut the 

 very minute egg of a sea-urchin into three parts, and reared 

 a larva from each of them. In another case he reared an 

 embryo from Ath of a sea-urchin's egg. Twin animals 

 may often be obtained from one ovum by producing a 

 separation of the first two cleavage-cells. Professor E. 

 B. Wilson produced quadruplets by shaking apart the 

 four-cell stage in the development of thelancelet. 



The second wonder is germinal continuity. These 

 germ-cells are not ordinary cells ; they are like the fertilized 

 ovum which gave rise to the parent. All the cells of the 

 body are continuous with the original fertilized ovum by a 

 succession of cell-divisions, but in the case of the germ-cells 

 the lineage is undiflerentiated. In many cases, scattered 

 throughout the animal kingdom, from worms to fishes, 

 the beginning of the lineage of germ-cells is demonstrable 

 very early, before the division of labour implied in building 

 up the body has more than begun. Even when this early 



