CLEAVACxE AND DEVELOPMENTAL PATTERN 597 



ferentiation is evidently possible, not only in absence of the normal cleav- 

 age pattern but in absence of any cleavage or nuclear division. Again it 

 appears that, even in forms with spiral cleavage, cleavage pattern is not 

 a fundamental factor in differentiation. 



CONCLUSION 



It is evident that early cleavage pattern and organismic developmental 

 pattern may be completely independent or more or less closely associated. 

 In some cases cleavage may perhaps be a factor in determining develop- 

 mental pattern, but more commonly developmental pattern plays some 

 part in determining cleavage pattern when the two are related. Even 

 in some forms with determinate cleavage more or less alteration of cleav- 

 age does not necessarily alter developmental pattern. At present it ap- 

 pears highly improbable that any organism is, in any strict sense, a mosaic 

 of self-differentiating cells or cell grou-s. At certain stages some parts may 

 be more independent than others, but capacity for more or less independ- 

 ent differentiation when they are isolated is not proof that they are in- 

 dependent in the intact organism. Moreover, the postulated "organ-form- 

 ing substances" do not exist. An organ is the product of an action system 

 or various systems in relation to others. A multicellular organ involves 

 orderly relations of cells and a definite spatial pattern. A particular sub- 

 stance may bring about differentiation of a cell or tissue; but how can it 

 form an organ? Trochoblasts of the annehd embryo or the presumptive 

 notochord of the ascidian may differ specifically from other parts, and 

 their specific constitutions may determine their differentiation; but a 

 prototroch or a notochord can develop only in relation to other parts. 

 The formative factors in development are interrelated action systems. 



Even in the most extreme cases early development approaches or attains 

 mosaic character only as regards certain parts, largely those giving rise 

 to temporary larval organs or apical or cephalic regions. Eggs of different 

 species differ widely as regards degree or stability of regional determina- 

 tion at the beginning of embryonic development; but in no case known 

 are all parts fixed and stable in their determination or differentiation, 

 even in relation to isolation alone. 



Once more it may be recalled that in many forms with relatively mosaic 

 embryonic development this is only one of various ways in which indi- 

 viduals of the species may originate. For example, spiral determinate 

 cleavage is characteristic of embryonic annehd development; but in many 

 annehd species individuals can develop from buds, from fissions, or from 



