S62 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tic of the adult polychete and oligochete. In the polychete this gradient 

 may be initiated by the coming-together ventrally of the active borders of 

 the somatic plate and in the oligochetes by the apposition ventrally of the 

 germ bands. 



The highly stable condition of the more apical regions in early stages 

 and the high capacity for reconstitution of the adults of many annelids 

 has sometimes been regarded as presenting a puzzling problem. However, 

 the most stably determined cells of the apical region, such as the trocho- 

 blasts and apical tuft cells, apparently take no part in postlarval develop- 

 ment, and even in the adult the head region is apparently fixedly deter- 

 mined. But in the embryo the Z)-quadrant and the somatoblasts derived 

 from it are totipotent for ectoderm and mesoderm and are capable of re- 

 constitution. They are no more determined or differentiated than post- 

 cephalic regions in the adult, perhaps less so. 



The most striking features of spiral cleavage are the definiteness of 

 cleavage pattern in a given species and the differences of pattern in rather 

 closely related species. However, cleavage pattern can be altered in vari- 

 ous ways, as will appear in following sections. The cleavage pattern is evi- 

 dently an expression of a cortical or general cytoplasmic pattern of some 

 sort in the egg, but the oblique form of cleavage apparently masks to some 

 extent the organismic pattern in early stages, so that it appears only grad- 

 ually and does not at first correspond in different cell groups. Apparently 

 the factors that determine cleavage planes oblique to the polar axis are not 

 very closely associated with organismic developmental pattern, but differ- 

 ences in cell size are probably related in some way to this pattern. The 

 gradual appearance of bilateral pattern, the lack of correspondence in the 

 bilateralities of different regions in earlier stages — for example, in the two 

 somatoblasts and their derivatives — and the gradual "adjustment" in the 

 course of development are difficult to account for in terms of a develop- 

 mental mosaic. The fact that isolated cells or cell groups are capable of 

 more or less differentiation in the same way as if other parts were present 

 does not prove that they are independent of other parts in the intact or- 

 ganism. 



Although spiral cleavage pattern is relatively stable in those groups in 

 which it appears, the occurrence of widely different cleavage patterns in 

 related groups seems not without significance, as indicating that cleavage 

 pattern and organismic pattern are not very closely related. Among the 

 turbellaria spiral cleavage appears only in polyclads. Cleavage of the acoe- 

 lous Polychoerus differs from the spiral type, according to Gardiner, and 



