868 PHYSIOLOGY. 



world, could take place. Haeckel has published his views not only 

 i'or scientific readers, but, through his popular publication, he, more 

 than any one else, made the discussion of the problems of evolution 

 and inheritance accessible to the public at large; and the literature, 

 scientific as well as unscientific, called forth by his efforts, for and 

 against this theory, is enormous. The scientific investigations, how- 

 ever, have failed to show as yet a single instance of a gastrula, or 

 its modification, developing into any other animal than one similar 

 to that from which it itself originated. On the other hand, it has 

 been conclusively shown that the physiological differentiation of the 

 cells constituting the blastoderm is established long before the germ- 

 inal layers are distinctly differentiated. We must, therefore, con- 

 clude that the lever for lifting the mystery of our phenomena is to 

 be applied at an earlier period than the already formed blastoderm. 

 The facts, which have accumulated within recent years, on the mor- 

 phology of the cell and its physiological manifestations during the 

 process of reproduction have, as we have seen above, been astonish- 

 ing. Particularly the nucleus has attracted the most attention, and 

 it has been shown very conclusively that the chromosomes of the 

 nuclei of the sexual cells are the principal factors in transmitting 

 the hereditary manifestations during reproduction. The most recent 

 studies, particularly those of Conklin, have revealed the fact, how- 

 ever, that the cytoplasm of the egg-shell also has a more highly dif- 

 ferentiated structure than was suspected. It has been conclusively 

 shown that many of the future organs are already mapped out in the 

 two-cell stage, and even in the unsegmentated ovum. 



It was only natural that these new discoveries should exercise 

 great influence upon the conception of evolution, and therefore a 

 new theory, embodying all the newest achievements, could be expected 

 to be received with favor. This new theory is suggested by De Vries 

 as the "mutation theory" and is founded upon the phenomena of the 

 cell-life. It is a theory of evolution of living organisms through 

 evolution of their germ-cells, and suggests, in the words of Conklin, 

 that similarities in the character and localization of the material sub- 

 stances of the egg must be the initial causes of all similarities or 

 homologies which appear in the course of development. Modifica- 

 tions of this germinal organization, however produced, are probably 

 the immediate causes of evolution; and if it is to be accepted as 

 probable that certain types of animals have been derived from others, 

 it is evident that such transformations might be accomplished far 

 more easily in the egg than in the adult. Eelatively slight modifica- 



