5S8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ent. In these cases the germ bands undergo less growth, and the embryo 

 is smaller than normal. 



According to these data, the Z)-quadrant, containing the polar plasms, 

 is potentially a whole Tubifex embryo, or even two embryos; but the 

 A-, B-, and C-quadrants normally play a part in embryo formation. In 

 their absence the parts normally formed by them are reconstituted by 

 cells originating from the Z)-quadrant. Evidently, as regards this quad- 

 rant, development is not a mosaic. Penners regards the polar plasms, 

 which are contained in the Z)-quadrant, as "organ-forming substances" 

 and as determining the potentialities of the two somatoblasts.^ It is, how- 

 ever, an interesting question whether this conclusion, if correct, means 

 anything more than that the polar plasms represent all the potentialities 

 of the Tubifex egg cytoplasm, except perhaps those of the entoderm. 

 What sort of formative substance can it be which gives rise to the entire 

 ectoderm and mesoderm of an embryo or of two embryos? Are the polar 

 plasms anything more than active, yolk-free parts of the egg cytoplasm? 

 Do they differ from the entodermal cytoplasm except in being yolk-free? 

 As yolk-free cytoplasmic aggregations, they may be primarily regions of 

 more intense metabolism than other parts and consequently more or less 

 dominant. 



Many other annelids and mollusks show more or less definite aggrega- 

 tions of cytoplasm apically or basally or both, but their fates in cleavage 

 differ. In some cases they are localized in the micromeres of the first 

 quartet, and new yolk-free aggregations appear apically in the macro- 

 meres preceding formation of each quartet of micromeres. In some forms 

 they disappear completely in early development. The case for specific 

 significance of the polar plasms does not appear entirely convincing. Un- 

 doubtedly the cells CD and D and the somatoblasts 2d and 4d differ in 

 some way from the other cells; but, since they are able to reconstitute a 

 whole embryo if a part of the entoderm is present, the difference seems to 

 be in the direction of less, rather than more, narrowly limited specificity. 



DUPLICATIONS IN OTHER ANNELIDS AND MOLLUSKS 



Duplications have been produced experimentally in the polychetes 

 Chaetopterus, Nereis, and SaheUaria and in the pelecypod Cumingia by 

 subjection of eggs to pressure, low temperature, high temperature, cen- 

 trifuging, anaerobiosis, and KCN during a definite period preceding cleav- 



' See also Schleip, 1914a, h, 1929; von Parseval, 1922. 



