536 GENERAL PHYSIOLOGY 



theory of Weismann and Roux seeks in the egg the causes of 

 the origin of differentiated daughter-cells from the cleavage, the 

 idea of Pfliiger and Hertwig finds the causes pre-eminently in 

 factors acting from outside upon the cells. While, according to the 

 one view, cells divide into unlike products from internal causes, 

 according to the other, external factors essentially produce the 

 unlikeness in continued division. Doubtless both views are correct 

 in this respect, and here is a point where a reconciliation is possible. 



From the above-developed idea of the mechanism of the 

 development and reproduction of the individual cell upon the 

 basis of the metabolic changes arising because of growth, it is 

 evident that internal and external causes of form-changes cannot 

 be separated from one another. The whole process of producing 

 and changing form is a compromise, a correlation of factors lying 

 within and without the cell. Because the cell as a result of the 

 characteristic quality of its living substance has the propertj'- of 

 taking in substances from the outside and of giving out substances 

 to the outside, the elementarj^ vital process, the metabolism, 

 represents a compromise between the internal and the external 

 factors, without which the life of the cell is impossible. But, 

 since with otherwise uniform external conditions the cell grows as 

 a result of the composition of its living substance, the relations 

 with the external factors become changed, so that the latter now 

 act in a manner different from before. Thus, at every moment of 

 time a different compromise is effected between the cell and the 

 medium, between the internal and the external factors, the ex- 

 pression of which is the change, the development, and finally, the 

 reproduction of the cell. Hence it is clear that the change of the 

 cell, or the variation of the products of its division, is not dependent 

 solelyupon its internal character, or the external factors; development 

 and reproduction are an expression of changes in the metabolic relations 

 betiveen cell and medium, conditioned hi/ groivth. 



The fundamental distinction between a single free-living cell 

 and an egg-cell developing into a cell-community consists wholly 

 in the fact that the daughter-cells arising by the division of the 

 unicellular organism become separated from one another immedi- 

 ateljr after the division, while in the development of the egg-cell 

 the daughter-cells that arise in segmentation remain in connection 

 with one another. In the unicellular organism, therefore, the 

 correlation between cell and medium always passes through the 

 same short cycle of changes ; in the division of the egg-cell, 

 however, this correlation changes in an entirely' new way with 

 each of the almost innumerable divisions. Hence it happens that, 



differentiated organs of the embryo." " His's principle of organ-forming germ- 

 regions is here applicable ; it has been here demonstrated that gastrulation is a 

 mosaic work. " From this the reader who is familiar with the ideas of preformation 

 and epigenesis will easily be able to decide for himself in how far Roux is a prefor- 

 matiouist and in how far not. 



