CLEAVAGE AND DEVELOPMENTAL PATTERN 555 



sible to account for the gradual replacement of the spiral and radial pat- 

 tern by a bilateral or asymmetric pattern except in terms of an ordering 

 and integrating factor, a wholeness of some sort which determines the 

 gradual appearance of the new pattern? It seems beyond the bounds of 

 probability that a pre-established harmony in a mosaic of independent 

 cells or cell groups resulting from spiral cleavage could be so complete as 

 to accomplish this result. Whatever the conditions determining spiral 

 cleavage, they are apparently different from those determining ventro- 

 dorsal or bilateral pattern. Even in the gasteropods, in which reversal of 

 cleavage is associated with reversal of asymmetry, the cleavage pattern 

 and the pattern of asymmetry are very different. Neither cell homologies, 

 nor regional homologies, nor precocious segregation throw any light on the 

 physiological factors concerned in spiral cleavage. 



A physiological gradient pattern, at least in the polar axis, is indicated 

 in some forms by differential susceptibility, differential dye reduction, and 

 differences in hydrogen-ion concentration, and the differential in rate of 

 cleavage also suggests a polar gradient (pp. 119, 545) ; but in many forms 

 apicobasal pattern is apparently not merely a quantitative gradient but 

 a pattern of more or less definitely localized material differences, some- 

 times directly visible in the living egg. This pattern, however, is appar- 

 ently in large part or wholly an incidental and secondary result of the 

 essential pattern and can be altered to an extreme degree by centrifuging 

 without affecting development in most forms with spiral cleavage. 



EMBRYONIC RECONSTITUTIONS IN FORMS WITH SPIRAL CLEAVAGE 



NEMERTEANS 



The nemerteans Cerehratulus lacteus and C. marginatus have a typical 

 spiral cleavage ; but development of isolated parts of eggs before and after 

 fertilization shows, first, that they are not primarily mosaics and, second, 

 that there is a progressive regional determination or stabilization with 

 early cleavages.'* Nucleated or nonnucleated pieces of unfertilized eggs 

 sectioned in various planes, can be fertilized, cleave like whole eggs and de- 

 velop into normal dwarf pilidium larvae. According to Wilson, however, 

 pieces less than about one-fourth of total egg volume do not give rise to 

 complete larvae. Here, as in other embryonic and postembryonic recon- 

 stitutions, scale of organization is decreased in the isolated pieces; and 

 in pieces below a certain size the scale may be larger than the piece, and 

 partial forms may result. 



■f E. B. Wilson, 1903c; Zeleny, 1904; Yatsu, 1910^, c. 



