CLEAVAGE AND DEVELOPMENTAL PATTERN 559 



age.'' According to Tyler, the usual condition resulting in duplication is 

 equal first cleavage with spindle at right angles to its normal position 

 but in, or parallel to, the equatorial plane of the egg. In centrifuged 

 Chaetopterus eggs, however, the polar lobe may be attached to the other- 

 wise smaller cell of an unequal first cleavage, and with incorporation of 

 the lobe into the cell the two blastomeres become equal. The two blasto- 

 meres resulting from equal cleavage have the potentialities of the CD- 

 blastomere of normal cleavage, and duplications result, as in Tubifex. 

 These vary in degree, some being clearly cruciate forms, the duplication 

 involving pretrochal, as well as posttrochal, regions; others, with only 

 posttrochal duplication. The apical organ is usually not duplicated, but 

 eyespots, mouth, and other organs may be. In general the completeness 

 of duplication is greater in the posttrochal, than in the pretrochal, region. 

 Both 1/2 blastomeres isolated following equal cleavage can develop ap- 

 parently normal trochophores. The pretrochal duplications indicate that 

 even in this region there may be some alteration in fates of some of the 

 cells in consequence of altered relations to other cells. The wide range in 

 degree of completeness of posttrochal dupHcation and the frequent de- 

 velopment of complete and separated posttrochal regions indicate that 

 the somatoblasts are totipotent for ectoderm and mesoderm of this region 

 and capable of reconstitution. 



ISOLATIONS AND TRANSPLANTATIONS IN ANNELID AND MOLLUSK EMBRYOS 



Isolation experiments with cleavage stages of the polychete Sahellaria 

 show that the formation of the apical tuft of cilia depends on the presence 

 of the first polar lobe and the C-cell; development of the posttrochal re- 

 gion, on the presence of D. Transplantations and unions of blastomeres 

 and of whole eggs show complete self-differentiation of all blastomeres. 

 Exogastrulae produced by treatment with alkahne isotonic NaCl show 

 completely independent development of ectoderm and entoderm with no 

 evidence of induction. However, duphcations result from KCN treat- 

 ment; evidently reconstitution of pattern is possible in the region from 

 which the somatoblasts form.^ 



After removal of the polar lobe of the two-cell stage of Ilyanassa the 

 second cleavage is equal; and the cell 4d, the mesoblast in normal develop- 

 ment, is the same size as other cells of the fourth quartet, instead of much 



7 Titlebaum, 1928; Tyler, 1930; Novikoff, 1939. 



*Novikoff, 1938,1939, and 1940, "Morphogenetic substances and organizers in annelid 

 development," Jour. Exp. Zool., 85. See also Hatt, 1931, 1932. 



