GERM LAYERS 45 



The nervous system develops from epithelium, but its cells, singly 

 or in groups, become imbedded in strands and masses of nerve fibers which 

 these same cells send out as processes. Thus little remains in the adult 

 to suggest that the brain or peripheral nerves come from a layer of cells 

 covering a surface, and the nervous system is therefore described as con- 

 sisting of nervous tissue. 



The voluntary muscles are formed from cells derived from the epithe- 

 lium of the mesodermic somites, but they develop as the somite breaks 

 up and its epithelial character is lost. The involuntary muscles are pro- 

 duced by a transformation of mesenchymal cells into elongated muscle 

 cells. For physiological reasons these two kinds of muscle, which are 

 of diverse origin and structure, are classed together as muscular tissue. 



The relation of the germ layers to the five fundamental tissues which 

 have now been recognized, is shown in the following summary. 



ORIGIN OF THE TISSUES FROM THE GERM LAYERS. 



The ectoderm produces: 



1. EPITHELIUM of the following organs: the skin (epidermis) including the cutane- 

 ous glands, hair and nails; the cornea and the lens; the external and internal ear; the 

 nasal and oral cavities, including the salivary glands, the enamel of the teeth and ante- 

 rior lobe of the hypophysis^ the anus; the cavernous and membranous parts of the male 

 urethra; together with that epithelium of the chorion which is toward the uterus and 

 of the amnion which is toward the embryo. 



2. NERVOUS TISSUE forming the entire nervous system, central, peripheral and 

 sympathetic. 



3. MUSCULAR TISSUE, rarely, as of the sweat glands, and iris. 

 The mesoderm produces: 



1. EPITHELIUM of the following four sorts: (i) epithelium of the urogenital organs 

 (except most of the bladder and the urethra) and the epithelioid cords of cells in the 

 suprarenal gland; (2)epithelium of the pericardium, pleurae, and peritoneum and the 

 continuation of this layer over the contiguous surfaces of amnion and chorion; (3) epi- 

 thelium lining the blood vessels and lymphatic vessels; and (4) epithelium lining the 

 joint cavities and bursae. 



2. MUSCULAR TISSUE, striated (voluntary), cardiac, and smooth (involuntary). 



3. MESENCHYMA, an embryonic tissue, which forms in the adult, connective and 

 adipose tissue, bone (including the teeth except their enamel), cartilage, tendon, and 

 various special cells. 



4. VASCULAR TISSUE, the cells of the blood and lymph, consequently the essential 

 elements of the lymph glands, red bone marrow and spleen. 



The entoderm produces: 



1. EPITHELIUM of the following organs: the pharynx, including the auditory 

 tube and middle ear, thyreoid and thymus glands; the respiratory tract, including 

 larynx, trachea, and lungs; the digestive tract, including the oesophagus, stomach, 

 small and large intestine, rectum, liver, pancreas, and the yolk-sac; and part of the 

 urinary organs, namely most of the bladder, the female urethra, and prostatic part 

 of the male urethra (including the prostate). 



2. NOTOCHORDAL TISSUE, which occurs in the nuclei pulposi. 



