598 PATTERNS AND PROBLEMS OF DEVELOPMENT 



experimentally isolated pieces of the adult body, in some species even 

 from a single segment. An ascidian can develop not only from an egg with 

 highly determinate cleavage and apparent regional cytoplasmic differ- 

 entiation but from buds of various origin, from isolated pieces of the 

 adult body, from small pieces of stolons, and in many species from aggre- 

 gations of cells, forming as other parts of the adult body undergo involu- 

 tion or degeneration. These forms of development may involve origin 

 of new polarities, symmetries, or asymmetries, and localizations of pri- 

 mordia in definite and orderly relations to the new axiate pattern; but an 

 organization and cleavage like that of the egg and early embryo are evi- 

 dently not essential in these forms of development. Development from 

 the egg is apparently the most highly specialized form of annelid and 

 ascidian development. Egg organization at the beginning of embryonic 

 development in such forms does not represent the real beginnings of indi- 

 vidual development. Only by comparative investigation and analysis of 

 the different forms of development can we hope to distinguish the funda- 

 mental factors in origin and development of individual pattern from those 

 incidental to a particular form of development. 



If annelid or ascidian embryonic development were a rigidly determined 

 mosaic, budding, fission, and reconstitution of pieces would be impossible 

 in later life. If dediiTerentiation of cells is involved in these forms of de- 

 velopment, the cells which dedifferentiate were not rigidly determined. 

 If we assume that certain cells remain undifferentiated and are activated 

 in some way in budding, fission, and reconstitution of pieces, those cells 

 were certainly not rigidly determined. 



In many organisms, even higher vertebrates, incapable, so far as we 

 know, of giving rise in adult life to new individuals by budding, fission, 

 or reconstitution of isolated parts, these forms of development are possible 

 in early embryonic stages and have proved valuable aids in physiological 

 analysis. It is, of course, far from true that every individual organism 

 originates from an egg, and some of the other ways in which individuals 

 originate may be more important than the egg and its development for 

 attainment of an adequate concept of origin and nature of developmental 

 pattern. 



