Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 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 t
Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 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-
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