CLEAVAGE AND DEVELOPMENTAL PATTERN 561 



lobe and the apical tuft probably results from an inhibition of development 

 following removal of the lobe rather than from presence in the lobe of a 

 specific tuft-forming substance. In any case, effects of removal of lobe in- 

 dicate that development in the forms concerned is not a mosaic. The rela- 

 tion of lobe or somatoblast and apical tuft suggests the possibility that the 

 first somatoblast may have some inducing action. Novikoff (1940, see 

 footnote 8, p. 559) maintains that a cytoplasmic substance present in 

 the polar lobe of Sabellaria directs development of the cell or cells into 

 which it enters and is to be regarded as both a morphogenetic substance 

 and an organizer. 



The first two cleavages of Patella are almost or quite equal; consequent- 

 ly, the Z)-quadrant cannot be distinguished in early stages, and 2d, sup- 

 posedly the first somatoblast, is no larger than other cells of the second 

 quartet. Isolated blastomeres cleave as if the other cells were present; but 

 partial forms, even those from 1/8 and 1/16 blastomeres, usually form closed 

 embryos and may gastrulate if entoblast cells are present. Isolated 1/4 

 blastomeres and isolated micromeres of the first quartet from all four 

 quadrants give rise to larvae with apical tuft, differing in this respect from 

 Dentalium, in which only the Z)-quadrant develops the tuft and only if the 

 first polar lobe is not removed. Isolated cells of the first quartet develop 

 according to their lineage into cells of the apical organ, cihated prototroch 

 cells, and other ectoderm cells. Apparently dorsiventral pattern is further 

 advanced in development at beginning of cleavage in Dentalium than in 

 Patella. 



GENERAL SUGGESTIONS CONCERNING SPIR.A.L CLEAVAGE 



The very general decrease basipetally in rate of cleavage in early stages 

 suggests an apicobasal gradient of some sort, and other evidences of such 

 a gradient have been found (pp. 119, 545). The fixity or stabihty of pat- 

 tern apparently undergoes some decrease basipetally. The regions of most 

 stable determination or specific constitution, as indicated by the isolation 

 experiments, represent apical or anterior regions of the larva and are essen- 

 tially head regions. In this respect the forms with spiral cleavage resemble 

 many other forms. Isolated apical regions of adult hydroids and heads of 

 planarians and annelids do not reconstitute other parts, but apical regions 

 or heads can reconstitute from lower levels. Essentially similar conditions 

 appear in sea-urchin embryos. 



At some time in annelid development a ventrodorsal gradient must ap- 

 pear, at least in the posttrochal region, for such a gradient is characteris- 



