674 LECTURE XXIX. 



He pointed out that the 1 land vertebrates are richer in sodium chloride 

 in proportion as the stage of development is young. This fact was illus- 

 trated particularly clearly by comparing the sodium chloride content of 

 the cartilage of embryo and that of later stages of development of the 

 same species. The older the animal, the lower sinks the content of 

 sodium and chlorine. There must be some reason why the cartilage of 

 the embryo is so rich in common salt. This fact is all the more striking 

 because the land is poor in this salt, potassium salts preponder- 

 ating. Typical inhabitants of the land, such as insects, have in 

 their bodies the elements sodium and potassium in about the same 

 proportion as that in their food. The remarkably high content of sodium 

 chloride in the cartilage of the early stages of development of verte- 

 brates which inhabit the land may be regarded, like the appearance 

 of the gill-slits and other similar phenomena, as an ancestral reminis- 

 cence. It is not at all strange that even the chemical composition 

 should indicate long-forgotten conditions. 



Not alone the development of the individual leads to such various 

 problems, but in later life also we meet with processes the nature of which 

 we can understand only very imperfectly. We usually assume that the 

 development of the entire organism can be traced back eventually to three 

 germ-layers. We can indeed distinguish these as regards their chemical 

 function and their construction even although one and the same layer, 

 as the ectoderm, may be of very heterogeneous construction. We can 

 imagine that the cells of each of these three germ-layers among themselves 

 have certain common characters so that eventually each individual cell 

 is in a position of replacing other cells which have resulted from the 

 same germ-layer and even to form these anew. It is not sufficient, how- 

 ever, to conceive that these three germ-layers correspond to three great 

 classes of different cells. They must, as we have already suggested, 

 all show common characteristics. To what extent the body-cells of 

 even adult organisms have the ability of changing their composition, 

 and thereby their function, is shown by the following experiment. 1 If 

 the crystalline lens is entirely extirpated from a water salamander, after 

 a short time there will be found in its place a newly formed transparent 

 structure which perfectly corresponds to the original lens. It is very 

 interesting to find that the new formation of the lens starts from the 

 epithelial cells of the iris. Now the cells of the latter are normally as 

 opaque as possible. Nevertheless these cells multiply after the removal 

 of the lens, and the pigment, which causes the opacity, disappears. In 

 this process comprehensive chemical processes must take place. The cells 



1 Cf. Vincenzo L. Colucci: Memor. della R. accad. delle scienze dell' inst. di Bologna, 

 Serie 5, T. 1, p. 593 (1890). Also G. Wolff: Arch. Entwicklungsmechanik der Organis- 

 men, 1, 380 (1896). 



