ONTOGENESIS 163 



the outer integument, the sense organs, and the 

 nervous system. From the endoderm develops the 

 digestive portion of the alimentary canal. From the 

 mesoderm develop all the other structures. Just 

 as the unicellular zygote became differentiated into 

 the blastula, and the latter into the gastrula, so the 

 germ-layers, which are primarily sheets of apparently 

 similar cells, differentiate into the most varied forms. 

 This is accomplished ultimately by the differentiation 

 of the cells themselves (histogenesis), as in the forma- 

 tion of muscle cells or nerve-cells, but such a change is 

 preceded by a shaping of rudiments of organs by the 

 differential growth of the germ-layers. Thus the 

 ectoderm, in the vertebrates, forms first a shallow 

 groove along the axis of the embryo, which becomes 

 deeper by the more rapid growth of the sides, until 

 the latter close over and meet above to form a tube, 

 the anterior end of which, by a further complication 

 of folds, flexures, and cell-growth becomes the em- 

 bryonic brain. The cells composing such a struc- 

 ture are apparently all alike in appearance. Not 

 until the organ is "blocked out" does histogenesis 

 begin. 



The tracing out of the relative times of appearance, 

 the structure, and mutual relations of the various 

 parts o'f the organism as they are transformed, one 

 from another, as well as the dynamic factors that 

 may control or alter these changes, constitutes the 

 special science of embryology. The details of these 

 later transformations vary greatly in different forms 

 of life. All animals, however, begin development 



