INTRODUCTION 23 



of animals had been investigated, and it had been 

 found that in their early development they all passed 

 through the same two stages. These stages are called 

 the blastula and gastrula, and presumably represent 

 the ancestors from which all animals have been 

 derived. The original form of Haeckel's law suggests 

 that these ancestors looked like blastulae or gastrulae 

 when they were adult, but no animals of this kind 

 have survived till the present day. In fact, if we 

 adopt the modification of Haeckel's law which was 

 advanced above, there is no need to suppose that 

 adult blastulae and gastrulae have ever existed; we 

 need only assume that the original ancestral organ- 

 isms from which all animals have been evolved 

 passed through these two stages in their development. 

 There is quite a large amount of variation in the 

 shapes assumed by the blastulae and gastrulae of 

 different animals, but we can imagine ideal forms 

 from which all the others can be derived by minor 

 modifications. The ideal blastula consists of a hollow 

 ball of cells, the walls of which are only cell-thick. 

 The hollow inside is called the blastula cavity, or 

 blastocoel. The ideal gastrula is also a hollow ball, 

 but differs from the blastula in two ways; the ball 

 is punctured, and the walls are thicker and consist 

 first of two layers of cells and later of three. The 

 hollow inside, together with the innermost layer 

 which lines it, is the primitive gut, and communicates 

 with the outside through the hole which punctures 

 the ball. This hole is called the blastopore, because 

 when it becomes visible as a little pore on the blastula 



