Chap. Ill] EPITHELIAL TISSUE 27 



1. The epithelial tissues. 3. The muscular tissues. 



2. The connective tissues. 4. The nervous tissues. 

 Such fluids as blood and lymph are frequently described as liquid 

 tissues. 



Origin of tissues. — It has been stated that the cell is the struc- 

 tural unit of the body, and in the beginning the body develops 

 from a single cell named the ovum. The ovum is developed in the 

 ovary and is made fertile by the entrance into it of a cell, known 

 as the spermatozoon formed in the testes of the male. After fertili- 

 zation or impregnation takes place, the cells divide and subdivide 

 until their number is enormously increased. 



Fig. 8. — Diagrams to illustrate Fertilization of an Egg-cell (Ovum) bt a 

 Sperm-cell (Spermatozoon). .4, e, nucleus of a matured egg-cell ; s, a sperm-cell 

 ready to enter. B, sperm-cell entered and transformed into sperm-nucleus (s). 

 C, sperm-nucleus and egg-nucleus united, fertilization complete. D, division lead- 

 ing to two-cell stage. (Bigelow.) 



The cells thus formed eventually arrange themselves in the form of 

 a membrane, blastoderm, which is composed of three layers. These 

 layers are known respectively as ectoderm, mesoderm, and entoderm. 



The ectoderm, or outer layer, forms the epidermis and the nervous 

 system. , 



The mesoderm, or middle layer, forms the circulatory and urino- 

 genital systems, also the muscles, bones, and other connective 

 tissues. 



The entoderm or inner layer forms the greater part of the alimen- 

 tary and respiratory tracts, also the liver, pancreas, and other 

 glands. 



EPITHELIAL TISSUE 



Epithelial tissue is composed entirely of cells united together 

 by adhesive matter or cement substance. The cells are generally 

 so arranged as to form a skin, or membrane, covering the external 

 surfaces, and lining the internal parts of the body. This mem- 

 brane is seen when the skin is blistered, the thin and nearly 



