V 



ii2 THE DARWINIAN THEORY 



which is the most remarkable fact in the whole of 

 embryology — is that they are descended from 

 Protozoa, may not we hope to find from the 

 study of early stages of development some hint 

 as to the mode of origin of Metazoa in the first 

 instance ? 



Let us consider, for example, the actual develop- 

 ment of Amphioxus. The egg divides into a 

 number of cells, which, instead of separating, remain 

 together and continue to divide again and again, 

 giving rise to the morula stage. The next stage is 

 the tubular condition, where the cells are arranged 

 regularly round a central cavity with an aperture at 

 each end. This is followed by the blastula stage, 

 which consists of a hollow ball, the outer cells of 

 which are furnished with cilia enabling the embryo to 

 swim freely. During later stages foldings take place, 

 caused by outgrowths in some places and depres- 

 sions in others, whereby the shape is gradually 

 altered. (Fig. 20.) 



Our present point is to ascertain whether these 

 earliest stages are possible ones ; whether there are 

 organisms which remain permanently in one of these 

 conditions — viz., (1) a single cell; (2) a heap of 

 similar cells ; (3) a hollow tube ; (4) a ciliated hollow 

 ball. 



As examples of the first condition, or that of a 

 single cell, the Monads may be taken. These are 

 among the most minute and the simplest of living 

 organisms, having an oval body, a nucleus, and a 

 flagellum. They are found in infusions of animal 

 and vegetable matter. An example of the second 



