68 DOCTRINE OF EVOLUTION 



fore the egg of the frog becomes an elHptical mass of 

 cells, it is at one time a double-walled sac enclosing a 

 central cavity; in this stage it is called a gastrula. 

 Tracing back the mode of its formation, we find that it is 

 produced from a hollow sphere of fewer cells that are 

 essentially alike; this stage also is so important that 

 the special term hlastula is applied to it. Still earlier, 

 there are fewer cells — 128 or thereabouts, 64, 32, 16, 

 8, 4, 2, and 1. In other words, the starting point in 

 the development of the frog is a single biological unit; 

 this divides and its products redivide to constitute the 

 many-celled blastula and the double-walled gastrula. 

 All the other animals we have mentioned begin like 

 the frog, as eggs which are single cells and nothing more ; 

 they too pass on to become blastulse and gastrulse, 

 similar to those of the frog in all essential respects, 

 particularly as regards the nature of the organs produced 

 by each of the two primary layers, and the mode of their 

 formation. Does the occurrence of blastulse and gas- 

 trulse and one-celled beginnings mean that the higher 

 animals composed of numerous and much differentiated 

 cells have evolved in company from two-layered saccular 

 ancestors which were themselves the descendants of 

 spherical colonies of hke cells, and ultimately of one- 

 celled animals ? 



Comparative anatomy has asserted that this is so, 

 as we have already learned, for it finds that adult ani- 

 mals array themselves at different levels of a scale 

 beginning at the bottom with the protozoa, continuing 

 on to the two-layered animals hke Hydra and jelly- 

 fish and sea-anemones, and then extending upwards to 

 the region of the more complicated invertebrates and 



