156 EVOLUTION OF TO-DAY. 



We can take yet another step in the development 

 and still find all animals to agree. We reach thus 

 the much-discussed gastrula stage. In its simplest 

 form the gastrula is a two-layered sac, open at one 

 end. The two layers of the sac are composed of the 

 small cells into which the ovum has divided. These 

 two layers are different from each other, for the 

 outer one is the body-wall, and is sensitive, while the 

 inner one corresponds to the digestive tract. The 

 opening of the sac is the mouth, and is believed to 

 correspond, partially at least, to the mouth of higher 

 animals. 



According to Haeckel, this gastrula is a universal 

 stage of development, and represents the common 

 ancestor of all animals. There has hardly been in 

 the history of science a more happy prediction, 

 founded on such insufficient data, as this gastrea 

 theory of Haeckel. At the time it was made, em- 

 bryology was a very young science, but compara- 

 tively few facts were known, and many of the then 

 accepted facts have since proved to be erroneous. 

 But Haeckel, finding that many animals which he 

 had been fortunate enough to study passed through 

 a stage resembling a two-layered sac, with his char- 

 acteristic boldness he made the prediction that this 

 stage would be found universal in the animal king- 

 dom, and concluded that it consequently represented 

 an animal, living in ages past, which was the com- 

 mon ancestor of the whole animal kingdom. And 

 it is marvellous how this prediction has little by little 

 been shown to be at least partially true. Taking a 

 survey of the whole field of embryology to-day, it is 



