CLEAVAGE AND DEVELOPMENTAL PATTERN 583 



sions are usually definitely related to the polar axis, those of the first two 

 cleavages usually passing through this axis but often without definite re- 

 lation to a median plane. In most animals with meroblastic cleavage 

 little or no constant relation between cleavage pattern and pattern of 

 development has been discovered. Evidently developmental pattern may 

 be entirely independent of particular cells. Even in sea-urchin develop- 

 ment, although ectoderm, entoderm, and mesenchyme develop normally 

 from particular cells or regions, any one of the three parts can develop as 

 either of the others in the earlier stages. Normally the cleavage appears 

 to be, at least to a considerable degree, determinate; but nothing is defin- 

 itively determined in early stages. Ectodermization of prospective ento- 

 derm, entodermization of prospective ectoderm, mesenchyme formation 

 from prospective ectoderm or entoderm, and development of prospective 

 mesenchyme as ectoderm or entoderm, all occur under experimental con- 

 ditions. Dorsiventrality and polarity may be altered and even obliterated, 

 although the cleavage pattern of earher stages may remain entirely un- 

 changed. Evidently there is no necessary relation between cleavage pat- 

 tern and developmental pattern here. In reconstitutions in adult multi- 

 cellular forms the cell is evidently not an essential factor in developmental 

 pattern ; the pattern is supercellular. It is becoming increasingly evident 

 that, even in the so-called "mosaic forms," the relation of cleavage pat- 

 tern and the cell to developmental pattern is often much less definite than 

 has been assumed. In fact, even these forms are evidence of "the in- 

 adequacy of the cell theory of development" (Whitman, Woods Hole 

 Biol. Led., 1893). Most botanists are agreed that developmental pat- 

 tern in plants is not primarily a mosaic of cellular units of organization 

 and function. Years ago De Bary said: "Die Pflanze bildet Zellen, nicht 

 die Zelle bildet Pflanzen." The progress of botanical research has led 

 many others to essentially similar conclusions. A recent expression 

 of the view that plant organization is determined by the organ or organ- 

 ism, not by the cell, is the summary of studies in size and form of ovaries 

 and fruits of Cucurbitaceae in the symposium paper by E. W. Sinnott 

 (1939, "The cell-organ relationship in plant organization," Growth, 

 Suppl.). 



EFFECTS OF CENTRIFUGAL FORCE ON CLEAVAGE AND 

 DEVELOPMENTAL PATTERN 



Visible granules regionally localized in eggs and regional differences in 

 appearance of cytoplasm have often been regarded as representing forma- 

 tive substances, but centrifuge experiments have shown that in most 



