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, 



