62 BIOLOGY 



layered embryo is complicated by the fact that the 

 ovum has to prepare for its future nutrition, and at the 

 same time arrange for its freedom from external influ- 

 ence by surrounding itself with smooth membranes 

 within which it may develop. 



Internal development goes on simultaneously with 

 external, and whether this development is simple or 

 complex depends on the complexity of the organism into 

 which it will grow, as the complexity is essentially due 

 to the fact that numerous organs and series of organs 

 are being formed simultaneously by a folding, fusing, 

 or new grouping of some of the cells. 



It is more or less impossible to give a general account 

 of the development of the individual from the three- 

 layered stage, as the peculiar characteristics of each 

 begin to show, and the development does not necessarily 

 follow on exactly the same lines. There are, however, 

 some generalisations in connection with Ontogeny of 

 which a brief account may be useful. 



From the frequency of the two-layered or gastrula 

 stage in the development of animals, Haeckel came to 

 the conclusion that the first stable form of a many- 

 celled animal must have been something like a gastrula. 

 He named this hypothetical ancestor a Gastraea ; and 

 this supposition seems to be supported to a certain 

 extent, when we find that some of the simplest many- 

 celled animals are little more than a gastrula, e.g. such 

 forms as Dicyema, Ehopalura, Salinella, and many of 

 the simplest forms of sponges. 



When we examine the whole animal kingdom, we find 

 that the simplest animals are single-celled, and the next 



