GROWTH AND DIFFERENTIATION OF THE EMBRYO 3 



GROWTH AND DIFFERENTIATION OF THE EMBRYO 



A multicellular embryo develops by the division of the fertilized ovum to 

 form daughter cells. These are at first similar in structure, and, if separated, any 

 one of them may develop into a complete embryo, as has been proved by the 

 experiments of Driesch on the ova of the sea urchin. The further development of 

 the embryo depends: (1) upon the multiplication of its cells by division; (2) upon 

 the growth in size of the individual cells; (3) upon changes in their form and 

 structure. 



The first changes in the form and arrangement of the cells give rise to three 

 definite plates, or germ layers, which are termed from their positions the ectoderm 

 (outer skin), mesoderm (middle skin) and entoderm (inner skin). Since the ecto- 

 derm covers the body it is primarily protective in function, but it also gives rise to 

 the nervous system through which sensations are received from the outer world. 

 The entoderm, on the other hand, lines the digestive canal and is from the first 

 nutritive in function. The mesoderm, lying between the other two layers, 

 naturally performs the functions of circulation, of muscular movement, and 

 of excretion; it also gives rise to the skeletal structures which support the 

 body.' While all three germ layers form definite sheets of cells known as 

 epithelia, the mesoderm takes also the form of a diffuse network of cells, the 

 mesenchyma. 



The Aniage. — ^This German word, which lacks an entirely satisfactory 

 English equivalent, is a term appKed to the first discernible cell, or aggregation of 

 cells, which forms any distinct part or organ of the embryo. In the broad sense 

 the fertilized ovum is the aniage of the entire adult organism; furthermore, in the 

 early cleavage stages of certain embryos it is possible to recognize single cells or 

 cell groups from which definite structures will indubitably arise. The term aniage, 

 however, is more commonly applied to the primordia that differentiate from the 

 various germ layers. Thus the thickening of the epithelium over the optic vesicle 

 is the aniage of the lens. 



Differentiation of the Embryo. — The developing embryo exhibits a progres- 

 sively complex structure, the various steps in the production of which occur in 

 orderly sequence. There may be recognized in development a number of com- 

 ponent mechanical processes which are used repeatedly by the embryo. The 

 general and fundamental process conditioning differentiation is cell multiplication 

 and the subsequent growth of the daughter cells. The more important of the 

 specific developmental processes are the following: (1) cell migration; (2) localized 

 growth, resulting in enlargements and constrictions; (3) cell aggregation, forming (a) 



