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 quite similar in structure, and, 

 if separated, in some animals each may develop into a complete embryo 

 (sea urchin; certain vertebrates). The further development of the em- 

 bryo depends: (i) 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 posi- 

 tions the ectoderm (outer skin), mesoderm (middle skin) and entoderm 

 (inner skin). Since the ectoderm covers the body, it is primarily pro- 

 tective 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 ex- 

 cretion ; it also gives rise to the skeletal structures which support the body. 

 While all three germ layers form definite sheets of cells known as epi- 

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

 mesenchyma. 



The Anlage. This German word, which lacks an entirely satisfactory 

 English equivalent, is a term applied to the first discernible cell, or aggre- 

 gation of cells, which is destined to form any distinct part or organ of the 

 embryo. In the broad sense the fertilized ovum is the anlage of the entire 

 adult organism; furthermore, in the early cleavage stages of certain em- 

 bryos it is possible to recognize single cells or cell groups from which defi- 

 nite structures will indubitably arise. The term anlage, however, is more 

 commonly applied to the primordia that differentiate from the various 

 germ layers. Thus the epithelial thickening over the optic vesicle is the 

 anlage of the lens. 



Differentiation of the Embryo. The developing embryo exhibits a 

 progressively complex structure, the various steps in the production of 

 which occur in orderly sequence. There may be recognized in develop- 

 ment a number of component mechanical processes which are used 

 repeatedly by the embryo. The general and fundamental process condi- 

 tioning differentiation is cell multiplication, and the subsequent growth of 

 the daughter cells. The more important of the specific developmental 

 processes are the following: (i) cell migration; (2) localized growth, resulting 

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

 (b) sheets, (c) masses; (4) delamination, that is, the splitting of single sheets 



