4 INTRODUCTION 



into separate layers; (5) folds, including circumscribed folds which produce 

 (a) evaginations, or out-pocketings, as the intestinal villi, (b) imaginations 

 or in-pocketings, as the intestinal glands. 



The production of folds, including paginations and imaginations, due 

 to unequal rapidity of growth, is the chief factor in moulding the organs and 

 hence the general form of the embryo. 



Differentiation of the Tissues. The cells of the germ layers that 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. 136). 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 develop- 

 ment of modified tissue cells from the undifferentiated cells of the germ 

 layers is known as histogenesis. During histogenesis the structure 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 

 functions of a given tissue cannot 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, after which no new cells are formed. 

 This takes place in the case of the nervous elements of the central nervous 

 system. 



Throughout life, tissue cells are undergoing retrogressive changes. 

 In this way the cells of certain organs like the thymus gland and meso- 

 nephros degenerate and largely disappear. The cells of the hairs and the 

 surface layer of the epidermis become cornified and eventually are shed. 

 Thus, normally, tissue cells may constantly be destroyed and replaced by 

 new cells. 



Cytomorphasis.Tbia series of changes an embryonic (undifferen- 

 tiated) stage; progressive functional specialization; gradual degeneration; 

 death and removal -which tissue cells experience is known by the term 

 cytomorphosis. 



Postnatal Development. Development does not cease at birth, but 

 continues until the adult stage is attained. Birth, itself, initiates ana- 

 tomical and physiological changes of profound influence on the body. 

 Throughout the growth period, with its uneven but steadily slowing 

 growth rate, comes a remoulding of the shape of the body and its parts. 

 During this time most of the organs lose in relative weight; the skeleton 



