72 THE LIVING WORLD. 



long-continued study of embryology that all multi- 

 cellular animals pass through a stage which embryol- 

 ogists have called a gastrula. A typical gastrula is 

 shown in Fig. 6. It is seen to consist of a cup made 

 of two layers of cells, one of which forms the outside 

 of the cup and the other the inside. When the two 

 layers are thus formed they assume different func- 

 tions ; the outer layer of cells becomes especially 

 connected with the powers of motion, sensation, and 

 the other functions directly connected with the outer 

 world. The inner cells being removed from direct 

 contact with the exterior, and being in a measure 

 protected, take for their function the duty of the 

 digestion of food which is supplied to them by the 

 outer layer of cells, the opening serving for both 

 mouth and anus. Now all animals show evidence of 

 having passed through some such stage as this gas- 

 trula, and the interpretation of the fact can mean but 

 one thing. If embryology repeats past history this 

 must mean that near the beginning of the develop- 

 ment of the aru'mal kingdom, there was a common 

 ancestor which corresponded in its fundamental feat- 

 ures with this gastrula stage, and further, that all 

 animals which at the present time show traces of this 

 stage in their development have descended from that 

 common ancestor. To the common ancestral stage 

 thus indicated, Haeckel gave the name Gastraea, a 

 name which is seen to correspond to the embryologi- 

 cal stage, the gastrula. This name we will retain in 

 the following discussion, although it is very probable 

 that the Gastraea, as Haeckel understood it, never 

 existed. The idea as Haeckel conceived it has 



