20 GENERAL CONCEPTIONS. 



The Constitution of Organs. Few organs are formed from a single germ-layer, 

 for as we find organs in the vertebrate body they usually consist of two parts, one 

 of which may be regarded as the part proper of the organ, upon which the per- 

 formance of its special function directly depends, and the accessory part, which 

 supplies the necessary physiological conditions for the functioning of the organ. 

 For example: in a salivary gland the actual work of secretion is performed by the 

 epithelial cells of the gland, but these cells cannot act unless they are supported 

 by connective tissue and supplied with blood and lymph, three conditions which 

 depend upon the mesoderm, and also supplied with nerves, a condition which 

 depends upon the ectoderm. By far the majority of organs have their functional 

 part produced from epithelium, and this epithelium may come either from the 

 original outer or inner germ-layer, as the case may be, or from the mesothelial 

 portion of the middle layer. But the organ, as a whole, requires for its comple- 

 tion the addition of other elements, as indicated in the example given. We find, 

 therefore, that there are no adult organs which are constituted solely by either the 

 ectoderm or entoderm, although there are organs the principal part of which may 

 come from one or the other of these germ-layers, but to complete the organ the 

 mesoderm must help. On the other hand, the mesoderm may form complete 

 organs by itself, or at least with no other aid from the other germ-layers than is 

 given by the supplying of nerve-fibers. Such purely mesodermal organs are illus- 

 trated by the spleen, the kidney, and the sexual glands. 



The Relations of Surface to Mass. 



However much the weight of an animal increases during its development, the 

 ratio of the free surface to the mass alters but slightly from the ratio established 

 when the embryo begins to take food from outside. It is only for convenience 

 that I express this law in this precise form; in reality, about it our knowledge is 

 scanty and our conceptions vague. According to a geometrical principle, when the 

 bulk of a body bounded by a simple surface increases, the surface enlarges less 

 than the mass in the simplest case of a cube, the surface increases as the square, 

 the mass as the cube, of the diameter. If in a cube of unit diameter one unit 

 of surface bounds one unit of mass, then in a cube of three units diameter nine 

 units of surface will bound twenty-seven units of mass; the proportion in the first 

 cube is 1:1, in the second 1:3. To maintain the proper proportion in the embryo, 

 simple enlargement is insufficient, therefore the surface increases by becoming more 

 and more irregular. The irregularities are characteristic of each organ and part, 

 and may be either large or microscopic. They may be conveniently grouped under 

 two main heads projections and invaginations. 



Projections are illustrated by the limbs, filaments of the gills in fishes, the 

 villi of the intestine, folds of the stomach in ruminants, etc. In every case the 

 projection is covered by an epithelium and has a core of mesodermic tissue. 



