2 INTRODUCTORY I 



In development two factors are obviously involved. One is 

 growth, or increase of volume, more correctly increase of mass; 

 the other is differentiation, or increase of structure; and, in 

 multicellular organisms, both these factors are accompanied by 

 division of the nucleus and the cell. 



Segmentation is the first sign, or almost the first sign, the 

 developing ovum gives of its activity; and this cutting up of 

 the egg-cell into parts, which marks the beginning, is also con- 

 tinued during the later stages of ontogeny, and goes on as long 

 as the life of the organism endures. 



Growth is especially characteristic of the embryonic and of 

 the adolescent organism. It occurs at different rates in the 

 different cells, and indeed the growth of a group of cells is in 

 itself often an act of differentiation. Growth may depend upon 

 the absorption of water or the assimilation of other substances, 

 and this may lead simply to an increase in the size of internal 

 cavities, as in the blastula of Echinoderms or the Mammalian 

 blastocyst; to an increase in the volume of the living proto- 

 plasm; or to the secretion of intracellular or intercellular 

 substances, either organic (for example, the notochordal vacuoles, 

 the matrix of cartilage and bone) or inorganic (the skeletal 

 spicules of Echinoderm larvae, Sponges, and Coelenterata). This 

 increase of mass is not only conditioned by the presence of food 

 in the form of substances found in the environment, but depends 

 on such external circumstances as temperature, atmospheric and 

 osmotic pressure, and so forth. 



But while the embryo is dividing up its material a material 

 which is already to a certain extent heterogeneous, composed, 

 for example, of protoplasm and deutoplasm or yolk while it is 

 increasing its mass, it is also undergoing a process of differentia- 

 tion; and, as even a superficial acquaintance with embryology 

 will inform us, one of the most characteristic features of 

 differentiation is that it occurs in a series of stages which 

 follow upon one another in regular order and with increasing 

 complexity. When segmentation has been accomplished some- 

 times, indeed, during segmentation certain sets of cells, the 

 germ-layers, become separated from one another. Each germ- 

 layer contains the material for the formation of a definite set of 



