36 GENERAL CONCEPTIONS. 



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. The accompanying table presents the principal 

 tissues classified according to the layers to which they belong. Or, as we may 

 say, according to their layership, a word which is proposed to indicate the mem- 

 bership of a given cell or tissue in the germ-layer to which it belongs. The layer- 

 ship of a cell is never changed after the differentiation of the three primitive 

 layers has been accomplished. There have been classifications of organs on the 

 layership basis published before, but inasmuch as organs usually contain cells 

 from two layers, we get a more correct presentation of the actual genetic rela- 

 tionship by confining our tabulation to the tissues. Leucocytes do not appear 

 in the table for the reason that their first origin is uncertain. Blood-vessels 

 arise very early, before the clear separation of the mesoderm and entoderm has 

 occurred. It is possible that they are entodermal. With these two limita- 

 tions the table presents our present knowledge. 



The Constitution of Organs. The layership of most organs is not simple, 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 sup- 

 ported 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 meso- 

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

 completion 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 com- 

 plete 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 meso- 

 dermal organs are illustrated by the spleen, the kidney, and the sexual glands. 



