1 6 HOW ANIMALS DEVELOP 



move large lumps of the pattern about, but we cannot 

 discover what caused the pattern in the first place. 

 But there is a second line of attack. We can 

 actually watch how the parts of a living organism 

 come into being and fit together. Nearly all organisms 

 start life as fertihzed eggs, though a few grow out 

 as buds from other organisms. Fertilized eggs are 

 very simple-looking, often apparently quite homo- 

 geneous lumps of living matter. They consist of a 

 watery jelly, the protoplasm, which contains a variable 

 amount of food-matter or yolk, and which also 

 encloses a little bag of special material which is the 

 kernel or nucleus. As we shall see, the jelly-like 

 protoplasm is not really as simple as it looks. But it 

 is at any rate much simpler than the adult animal, 

 which consists of very large numbers of cells, of 

 several different kinds, arranged in various ways to 

 build up the different organs. During the increase 

 in complexity as the egg develops into the adult the 

 spatial pattern of the animal arises. In the early 

 stages it is fluid and unfixed; we can describe its 

 gradual unfolding, make experiments which alter it, 

 study its genesis and causation. 



The study of development, or embryology, because 

 it offers the possibility of finding out how the most 

 fundamental characteristic of living things, their 

 organization, comes into being, has always been of 

 compelling interest to everyone who has been con- 

 cerned with the position of living things in the 

 general philosophical scheme. Nearly all biological 

 philosophers, from Aristotle to the present day, have 



