The Ascent of Man 161 



has from the first a certain individuality, with peculiar charac- 

 teristics which are all its own. This is expressed by the some- 

 what difficult word specificity, which just means that every 

 species is itself and no other. So in the development of the 

 human embryo, while there are close resemblances to the embryos 

 of apes, monkeys, other mammals, and even, at earlier stages 

 still, to the embryos of reptile and fish, it has to be admitted that 

 we are dealing from first to last with a human embryo with pecu- 

 liarities of its own. 



Every human being begins his or her life as a single cell a 

 fertilised egg-cell, a treasure-house of all the ages. For in this 

 living microcosm, only a small fraction ( T 7 ) of an inch in diame- 

 ter, there is condensed who can imagine how? all the natural 

 inheritance of man, all the legacy of his parentage, of his ances- 

 try, of his long pre-human pedigree. Darwin called the pinhead 

 brain of the ant the most marvellous atom of matter in the world, 

 but the human ovum is more marvellous still. It has more possi- 

 bilities in it than any other thing, yet without fertilisation it will 

 die. The fertilised ovum divides and redivides; there results a 

 ball of cells and a sack of cells; gradually division of labour be- 

 comes the rule; there is a laying down of nervous system and 

 food-canal, muscular system and skeleton, and so proceeds what 

 is learnedly called differentiation. Out of the apparently simple 

 there emerges the obviously complex. As Aristotle observed 

 more than two thousand years ago, in the developing egg of the 

 hen there soon appears the beating heart ! There is nothing like 

 this in the non-living world. But to return to the developing 

 human embryo, there is formed from and above the embryonic 

 food-canal a skeletal rod, which is called the notochord. It thrills 

 the imagination to learn that this is the only supporting axis that 

 the lower orders of the backboned race possess. The curious 

 thing is that it does not become the backbone, which is certainly 

 one of the essential features of the vertebrate race. The noto- 

 chord is the supporting axis of the pioneer backboned animals, 



