GERM-LA YERS. 



35 



The Ccelom. The coelom is the primitive body-cavity of the embryo. It 

 arises as a space in the mesoderm. As soon as this space has appeared we find 

 that the cells of the mesoderm, which bound it, assume an epithelial character, con- 

 sequently the mesoderm, after the coelom has appeared, consists of mesenchyma 

 and of an epithelial layer bounding the coelom. This epithelial layer is called the 

 niesothelium. The mesoderm, therefore, differs fundamentally from the ecto- 

 derm and entoderm by this peculiarity, that it comprises both an epithelial and 

 a non-epithelial portion. Both portions play very important roles in the pro- 

 duction of the various tissues and organs of the body. There is another respect 

 also in which the mesoderm differs from the other germ-layers, for we find that it 

 increases in volume and in complexity as we ascend from the lower to the higher 

 types of animals, or as we pass from the embryo toward the adult condition, 

 more than does either the outer or inner germ-layer. 



CLASSIFICATION OF THE TISSUES. 



(A) ECTODKRMAL. 



Epidermis. 



a. epidermal appendages, 



b. lens of eye. 

 Epithelium of 



a. cornea, 



b. olfactory chamber, 



c. auditory organ, 

 r/. mouth 



(oral glands), 

 (enamel organ), 

 (hypophysis), 

 e. anus, 



f. chorion, 



foetal placenta, 



g. amnion. 

 Nervous system. 



a. brain, 



optic nerve, 

 retina, 



b. spinal cord, 



(B) MESODKRMAL. 



1. Mesothelium. 



a. epithelium of 



peritoneum, 

 pericardium, 

 pleura, 

 urogenital organs. 



b. striated muscles. 



2. Mesenchyma. 



a. connective tissue, 



smooth muscle, 

 pseudo-endothelium, 

 fat-cells, 

 pigment cells, 



b. blood, 



c. blood-vessels, 

 </. lymphatics, 



e. spleen, 



f. supporting tissues, 



cartilage, 

 bone, 



g. marrow. 



d. neuraxons. 



(C) ENTODHRMAL. 



1. Notochord. 



2. Epithelium of 



a. digestive tract, 



oesophagus, 



stomach, 



liver, 



pancreas, 



small intestine, 



yolk-sack, 



large intestine, 



caecum, 



vermix, 



rectum, 



allantois (bladder), 



b. pharynx, 



Eustachian tube, 



tonsils, 



thymus, 



parathyroids, 



thyroid, 



c. respiratory tract, 



larynx, 



trachea, 



lungs. 



The Specific Quality of the Germ-layers. Each germ-layer has its specific 

 and exclusive function in the production of tissues, giving rise only to the tissues 

 which are proper to it, and never to the tissues which are proper to either of the 

 other layers. We must, indeed, so far as our present knowledge goes, regard 



