CHAPTER II. 



THE ADULT TISSUES -OF THE HUMAN BODY. 



From the fertilized human egg cell there develops a body composed of 

 cells, differing widely from one another in details of structure and func- 

 tion, together with certain non-cellular structures produced and nourished 

 by cell activity. 



The tissues of the body necessarily vary in structure with the stages in 

 the development of the individual. We are concerned directly with the 

 human tissues as they exist in the adult, but will find it profitable to refer 

 briefly to their development within the uterus. Several classifications of 

 tissue are in vogue, the one here adopted being taken from Stohr. 



Fig. 9. Segmentation of the ovum (egg), and formation of the germ-layer in the 

 rabbit. (Lewis & Stohr, Histology.) A, two-cell stage; B, four-cell stage; C, mor- 

 ula. D-H, cross-sections of later stages. Ect., ectoderm ; Ent., endoderm ; Mes., meso- 

 derm. In G, the medullary or neural groove is plainly visible at the top, and in H, 

 the edges of the groove are about to unite to form the medullary tube. 



