INTRODUCTION 



cords, (b) sheets, (c) masses; (4) delamination, i. e., the splitting of single sheets into 

 separate layers; (5) folds, including circumscribed folds which produce (a) evagina- 

 tions, or out-pocketings, e. g., the intestinal vilH, (b) invaginations, or in-pocket- 

 ings, e. g., the intestinal glands. 



The production of folds, including evaginaiions and invaginations, due to un- 

 equal rapidity of growth, is the essential factor in moulding the organs and hence the 

 general form of the embryo. 



Differentiation of the 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. 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 mesen- 

 chymal 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 cannot give rise to cells of any other type. In tissues like 

 the epidermis, certain cells retain their primitive embryonic characters throughout 

 Ufe, 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 t}pe, 

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

 nervous elements of the centra] nervous system. 



Throughout Hfe, tissue cells are undergoing retrogressive changes. In this 

 way the cells of certain organs like the thymus gland and mesonephros degenerate 

 and largely disappear. The cells of the hairs and the surface layer of the epider- 

 mis become cornified and eventually are shed. Thus, normally, tissue cells may 

 constantly be destroyed and replaced by new cells. 



The Continuity of the Germ Plasm.— According to this important conception 

 of Weismann the body-protoplasm, or soma, and the reproductive-protoplasm 

 differ fundamentally. The germinal material is a legacy that has existed since 

 the beginning of hfe, from which representative portions are passed on intact 

 from one generation to the next. Around this germ plasm there develops in 

 each successive generation a short-hved body, or soma, which serves as a vehicle 

 for insuring the transmission and perpetuation of the former. The reason, there- 



