20 INTRODUCTION. 



mation of a considerable variety of tissues, grouped in various ways 

 to form organs or systems of very different kinds of utility to the 

 whole organism. This divergent development is known as differen- 

 tiation and results in a specialization of the different parts of the 

 body. Its study constitutes embryology, but it will make the com- 

 prehension of histology easier if some of the simpler and broader facts 

 derived from a study of development are first briefly summarized. 



A new individual arises through the detachment of a single cell, 

 the ovum (Fig. 1), from the parent organism. This cell divides 



FIG. 1. 



Section of human ovum and its immediate surroundings within the ovary. (Nagel.) a, zona 

 pellucida ; b, cytoplasm of the ovum ; c, granules and globules of stored food materials 

 within the cytoplasm, collectively known as the metaplasm or deutoplasm; d, germinal 

 vesicle or nucleus of the ovum containing, in this case, two germinal spots or nucleoli ; 

 e, zone of epithelial cells immediately surrounding the ovum ; /, cells of the discus pro- 

 ligerus ; g, perivitelline spaces separating the zona pellucida from the cytoplasm of the 

 ovum. 



into two cells, which, even at this stage of development, differ 

 slightly from each other. These daughter-cells in turn divide in 

 two, and this process of division is continued, each cell giving rise 

 to two new cells, until a considerable aggregate of cells has resulted 

 (Fig. 2). Then the cells assume a definite arrangement into layers. 

 Some become disposed in a superficial layer enclosing the rest of 

 the cells and a body of fluid. This layer is called the primitive 

 ectoderm. The remaining cells accumulate in an irregular laminar 

 mass beneath the primitive ectoderm at the site of the future em- 

 bryo. This mass of cells is the primitive entoderm. Thus, at 



