GROWTH AND DIFFERENTIATION OF THE EMBRYO 13 



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

 (outer skin), mesoderm (middle skin) and entoderm (inner skin). In function 

 the ectoderm, as it covers the body, is primarily protective, and 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 per- 

 forms the functions of circulation, of muscular movement and of excretion; it 

 gives rise also 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 Anlage. This German word is the term applied to the first ag- 

 gregation of cells which will form any distinct part or organ of the embryo. 

 The various anlages are differentiated from the germ-layers by a process of un- 

 equal growth. At points where multiplication of the cells is more rapid than in the 

 circular area surrounding them, outgrowths or ingrowths of the germ-layer will 

 take place. The outgrowths or evaginations are illustrated by the development 

 of the finger-like villi from the entoderm of the intestine ; ingrowths or invagina- 

 tions by the formation of the glands at the bases of the villi. According to Minot, 

 the development of evaginations and imaginations, due to unequal rapidity of growth, 

 is the essential factor in moulding the organs, and hence the body of the embryo. 



Differentiation of Tissues. The cells of the germ-layers which form 

 organic anlages may be at first alike in structure. Thus the evagination 

 which forms the anlage of the arm is composed of a single layer of like 

 ectodermal cells, surrounding a central mass of diffuse mesenchyma (Fig. 

 131). Gradually the ectodermal cells multiply, change their form and structure 

 and give rise to the layers of the epidermis. By more profound structural changes 

 the mesenchymal cells also are transformed into the elements of connective tissue, 

 tendon, cartilage, bone and muscle, aggregations of modified cells which are known 

 as tissues. The development of modified tissue cells from the undifferentiated 

 cells of the germ-layers is known as histogenesis. During histogenesis the struc- 

 ture and form of each tissue cell are adapted to the performance of some special 

 function or functions. Cells which have once taken on the structure and func- 

 tions of a given tissue can not give rise to cells of any other type. In tissues like 

 the epidermis, certain cells retain their primitive embryonic characters throughout 

 life and, by continued cell-division, produce new layers of cells which are later 

 cornified. In other tissues all of the cells are differentiated into the adult type 

 and, during life, no new cells are formed. This takes place in the case of the 

 nervous elements of the central nervous svstem. 



