44 THE PHYSICAL KINSHIP 



is by a simple study of the individual evolution of 

 its members. Each animal repeats in its individual 

 evolution the evolution of its species. This re- 

 capitulation is not always complete is, in fact, 

 frequently vague, sometimes circuitous, and often 

 broken or abbreviated. Processes requiring origin- 

 ally centuries or thousands of years to accomplish 

 are here telescoped into a few months, or even 

 days. It is not strange that the process is im- 

 perfect. But so firmly is the belief in the cor- 

 respondence of ontogeny and phylogeny fixed in 

 the minds of modern biologists that, in determining 

 the classification and affinities of any particular^ 

 animal, more reliance is placed on the facts of{ 

 embryology than on those of adult structure. 



The first thing that an animal becomes after it 

 is an egg unless it is a one-celled animal, in 

 which case it remains always an egg is two cells ; 

 these two cells become four; these four become 

 eight; and so on, until the embryo becomes a 

 many-celled ball, consisting of a single layer of 

 cells surrounding a fluid interior. A dimple forms 

 in the cell layer on one side of this ball, and, by 

 deepening to a hollow, changes the ball into a 

 double-walled sac. This is the gastrula the per- 

 manent structure of the sponges and celenterates, 

 and an (almost) invariable stage in the larval deve- 

 lopment of all animals above the sponges and 

 celenterates. The gastrula becomes a worm (or 

 an insect or a fish through the worm) by elongation 

 and enlargement, and by the development of the 

 endoderm, which is the inner layer of the cell wall, 



