GERM-LA YERS. 



19 



from the embryo toward the adult condition, more than does either the outer or 

 inner germ-layer. 



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 most, at 

 least, of the cells in the germ-layers as originally wholly indifferent as individual 

 cells. But we must, nevertheless, not forget that, as members of a germ-layer, 

 their potential fate is already restricted. It is probable, if we could successfully 

 transplant an undifferentiated cell from one germ-layer to another, that it could 

 take part in the production of the tissues proper to that layer. But it is further 

 probable that this would be impossible after the differentiation of the cells in any 

 layer had fairly begun. After a cell has become definitively a member of one of 

 the germ-layers, it probably never migrates to join another layer. The accompany- 

 ing table presents the principal tissues classified according to the layers to which 

 they belong. There have been classifications of organs on the germ-layer basis 

 published before, but inasmuch as organs usually contain cells from two layers, we 

 get a more correct presentation of the actual genetic relationship by confining our 

 tabulation to the tissues. Blood-vessels and blood-cells arise very early, before the 

 clear separation of the mesoderm and entoderm has occurred. It is possible that they 

 are entodermal. With these two limitations the table presents our present knowledge. 



(A) ECTODERMAL. 



Epidermis. 



a. epidermal appendages, 



b. lens of eye. 

 Epithelium of 



a. cornea, 



b. olfactory chamber, 



c. auditory organ, 



d. mouth 



(oral glands), 

 (enamel organ), 

 (hypophysis), 



e. anus, 



/. chorion, 



fetal placenta, 

 g. amnion. 

 Nervous system. 



a. brain, 



optic nerve, 

 retina, 



b. spinal cord, 



c. ganglia, 



d. neuraxons, 



e. chromaffme cells. 



CLASSIFICATION OF THE TISSUES. 



(B) MESODERMAL. 



1. Mesothelium. 



a. epithelium of 



peritoneum, 

 pericardium, 

 pleura, 

 urogenital organs, 



b. striated muscles. 



2. Mesenchyma. 



a. blood-cells (red and white), 



b. blood-vessels, 



c. connective tissue, 



cellular reticulum, 

 smooth muscle, 

 pseudo-endothelium, 

 fat cells, 

 pigment cells, 



d. lymphatics, 



e. spleen, 



/. supporting tissues, 



cartilage, 



bone, 

 g. marrow of bone. 



(C) ENTODERMAL. 



1. Notochord. 



2. Epithelium of 



a. digestive tract, 



oesophagus, 



stomach, 



liver, 



pancreas, 



small intestine, 



yolk-sac, 



large intestine, 



caecum, 



vermix, 



rectum, 



allantois (bladder), 



b. pharynx, 



Eustachian tube, 



tonsils, 



thymus, 



parathyroids,' 



thyroid, 



c. respiratory tract, 



larynx, 



trachea, 



lungs. 



