26 HISTOLOGY. 



derm at the mouth, anus, and urogenital opening. The line of separation 

 is not that of transition from skin to mucous membrane, but is indicated 

 by the transient membranes (the oral and anal plates) found in young 

 embryos. Nothing in the adult remains to show where the layers join. 



ORIGIN OF THE TISSUES FROM THE GERM L*AYERS. 



The ectoderm produces: 



1. EPITHELIUM of the following organs: the skin, including its 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 an- 

 terior 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 fetus. 



2. NERVE TISSUE forming the entire nervous system, central, peripheral 

 and sympathetic. 



3. MUSCLE TISSUE, rarely, as of the sweat glands, and perhaps also some 

 muscle fibers of the iris. 



The mesoderm produces: 



1. EPITHELIUM of the following structures: the urogenital organs except 

 most of the bladder and the urethra; the pericardium, pleurae, and peritonaeum 

 and the continuation of this layer over the contiguous surfaces of amnion and 

 chorion; the blood and lymphatic vessels; and the joint cavities and bursae. 



2. MUSCLE 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 auditor}' 

 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 fetal yolk sac; and part 

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

 prostatic part of the male urethra. 



2. NOTOCHORDAL TISSUE, which disappears (?) in the adult. 



EPITHELIA. 



Epithelium has already been defined as a layer of cells covering an 

 external or an internal surface of the body, having one side therefore free, 

 and the other resting on underlying tissue. Epithelia differ from one 

 another in embryonic origin, in the shape of their cells, in the number 

 of layers of cells of which they are composed, and in the differentiation 

 of these cells. All of these features should be recorded in any complete 

 description of an epithelium, and, except the origin, something of each 

 is to be observed in a single specimen. These four characteristics may 

 be considered in order. 



