130 ELEMENTS OF BIOLOGY 



Origin of Cell Layers. In order to understand the major 

 principles of animal organization in Metazoa it is necessary first of 

 all to be familiar with the beginnings of form as they appear when 

 the animal is developing from the egg. A fertilized egg undergoing 

 the changes that result in an adult is called an embryo and the de- 

 velopmental history of an adult is termed its ontogeny. The study 

 that treats of development is termed embryology. The ontogeny of 

 all multicellular animals includes some features that are universal; 

 hence the adults of all animals exhibit some common anatomical 

 principles. The fertilized egg, by repeated divisions, gives rise to a 



BLASTOCOELE 



Fig. 77. — Diagrammatic section throujjh the blastula stage in the development of 



an echinoderm (starfish). 



considerable number of cells, which in the simplest cases assume the 

 form of a hollow sphere (Fig. 77), resembling the shape of a volvox 

 colony. This stage of the embryo is known as the blastula. An 

 organism that in its adult stage is composed of the derivatives of a 

 single layer of cells would be described as a monoblastic ani.\l\l. 

 No adult monoblastic Metazoa occur, but all animals pass through 

 such a stage during their ontogeny. Some animals consist of the 

 derivatives of two layers of cells, that is, are diploblastic. The for- 

 mation of two layers from the single layered blastula of the embryo 

 is accomplished by the inversion of one hemisphere into the other to 

 form a cup-shaped structure consisting of an inner layer or endo- 

 derm and an outer layer, or ectoderm^ the cell layers being con- 



