GASTR^A THEORY. 49 



stage of development, Haeckel compares to the 

 gdstrula larva of many of the lowest animals the 

 form value of which is that of a sac with a single 

 opening the wall consisting of two layers an 

 exoderm covered with vibratile cilia, and an entoderm 

 lining the cavity, while the single opening is the 

 protostoma or primitive mouth. The lowest animals, 

 such as sponges and the simplest polyps, continue 

 throughout life in this two-leaved form ; but the 

 ovum of man and the mammifera rapidly passes 

 through it towards the higher stages of development 

 proper to them. 



To the correspondence in form value between the 

 gastrula larva and the ovum of the higher animals 

 in the stage of development under notice, Haeckel 

 applies his biogenetic fundamental law, thus : Man, 

 and all other animals which pass through a stage 

 of development in which their body consists of an 

 exoderm and an entoderm, must have descended from 

 a primaeval stem-form, the whole body of which con- 

 sisted throughout life of only two different strata of 

 cells or germinal layers, similar to the lowest zoo- 

 phytes of the present day. We are, therefore, he 

 continues, justified in admitting that in early times 

 such a common stem-form did exist ; and to this 

 hypothetical being, which he considers full of 

 significance, Haeckel gives the name of GASTR^A. 



In all animals, except the zoophytes referred to, 

 the two primary germinal layers of the blastoderma 

 subdivide in the further course of development into 



E 



