THE ARGUMENT FROM EMBRYOLOGY 113 



condition, a heap of similar cells, is found in Pan- 

 dorina, a colony of similar flagellate cells, all alike 

 and living together in a common capsule. The third 

 condition is found in Salinella, one of the most 

 recent discoveries, and one of the most remark- 



Fig. 20. 



Amphioxus, showing early stages in development. 



I. to IV. — Segmentation of the egg. 



V. — Tubular stage, with an opening at each end. 



VI.- — Blastula stage, consisting of a hollow ball of cells. 



The adult Amphioxus is shown at the top of the figure. 



able animals known, which is found in water con- 

 taining 2 per cent, of salt. This organism consists 

 of a tube, open at both ends, the wall of which is 

 formed of a single layer of cells. The fourth con- 

 dition, that of a hollow ball or blastula, is re- 

 presented by Volvox, the well-known fresh-water 



H 



