Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 REPRODUCTION AND DEVELOPMENT 265 FERENTiATioN. {b) Cell clivisions, mechanical foldings of the cell layers, and the migration of cells are in normal development care- fully and accurately adjusted to the time of difTerentiation of the several types of cells, so that development is an orderly process. Cell divisions and mechanical changes in position of the ceils, and differentiation are, however, independent processes. Experimentally one may int
Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 REPRODUCTION AND DEVELOPMENT 265 FERENTiATioN. {b) Cell clivisions, mechanical foldings of the cell layers, and the migration of cells are in normal development care- fully and accurately adjusted to the time of difTerentiation of the several types of cells, so that development is an orderly process. Cell divisions and mechanical changes in position of the ceils, and differentiation are, however, independent processes. Experimentally one may interfere with, and prevent cell divisions without arresting the chemical changes of differentiation. With these facts of the normal concurrence of differentiation and changes in position of the cells and cell layers in mind, the description to follow will be largely confined to descrip- tions of change in form dur- ing development. Cleavage. The first pe- riod of development is marked f~r'^?tfVi!^^B5T ^^^^^^^^^'-^ by a series of cell divisions and is known as the cleavage period. Division into two, then into four, then into eight Fig. 178.—The blastula stage of the cepha- - ,, , . lochordate, Amphioxus. and mere ceils, results m a hollow sphere of cells known as the blastula (Fig. 178). Essentially this process takes place in all forms of animals, but with consider- able modification in some. The cavity of the blastula is called the BLASToccELE. The truly spherical blastula of the lower invertebrates resembles the general structure of the colonial protozoon, volvox, which was described elsewhere (p. 89; p. 90). Biologists permit themselves to speculate as to whether this resemblance indicates that the ancestor of the Metazoa was a colonial protozoon of the struc- tural type of volvox, or whether the resemblance is due to the possibility that this sort of hollow sphere is a necessary stage in the development of an adult animal regardless of its ultimate ancestry. Gastrulation.
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