CLEAVAGE AND DIFFERENTIATION. 



19 



germs of organs spread out over a flat surface, and conversely, 

 that every point of the germinal disk is found again in a later 

 organ, I call the Principle of Organ-forming Germ-regions {or- 

 ganbildende Keimbezirke)^ This doctrine has been denied in its 

 totality by some authors, but, although it is still the subject of 

 much controversy, the evidence is accumulating that with certain 

 modifications it is true of a considerable number of animals be- 

 longing to several different types. The fact that, under unusual 

 or "abnormal" conditions, regions which would have developed 

 into certain parts develop into others is not a contradiction of 

 the entire principle, though it does limit its causal significance. 



Accepting the principle of His as true in certain cases, the 

 relation of cleavage to these " germ regions " might conceivably 

 be of two kinds; cleavage planes might follow the lines of 

 separation between these regions, in which case there would 

 follow a definite form of cleavage, each blastomere being 

 destined to give rise to definite organs or parts of the embryo; 

 or cleavage planes might cut across these regions indiscrimi- 

 nately, in which case an indefinite and inconstant form of 

 cleavage would probably result. Of course, if one does not 

 accept the principle of His, a third alternative is possible and is, 

 in fact, imperative, viz., cleavage is a mere sundering of homo- 

 geneous materials and every blastomere at the time of its 

 formation is like every other blastomere. 



The first of these alternatives has been presented in what is 

 commonly called the "mosaic theory" of Roux;i the second 

 in what might be called the " organization theory " of Whit- 

 man ;2 the third in what I venture to call the "homogeneity 

 theory " of Driesch.^ Disregarding for the present the causes 

 of differentiation and viewing merely its results, it is probable 

 that each one of these theories is true in certain cases. The 

 study of cell-lineage has shown that in any given species among 

 annelids, mollusks, ascidians, nematodes, and probably among 

 ctenophores, turbellarians, rotifers, and Crustacea each blasto- 



^ Roux, W., " Beitrage zur Entwicklungsmechanik des Embryo," Nr. V, 1888. 



2 Whitman, C. O., " The Inadequacy of the Cell-Theory of Development," 

 Biological Lectures, Wood's Holl, 1S93. 



3 Driesch, H., " Entwicklungsmechanische Studien," I-VI, Zeit. wiss. Zool., 

 Bde. 53, 55, 1891-93, 



