CLEAVAGE AND DEVELOPMENTAL PATTERN 579 



partial forms and concluded that an isolated blastomere gives rise only 

 to those parts which develop from it in intact embryos, that the cyto- 

 plasmic regions visibly distinguishable represent organ-forming sub- 

 stances, and that ascidian development is a mosaic. However, both Conk- 

 lin and Chabry found that the sensory primordium might appear in right 

 or left partial forms, and Conklin regarded this as a regulation. Develop- 

 ment of one to three papillae in half-larvae, three being the number in 

 wholes, was noted by G. A. Schmidt (193 1) and regarded as indicating 

 regulation. Development of isolated blastomeres was also studied by Ber- 

 rill (1932), who agrees with Conklin; but later it was found that in half- 

 larvae of Ascidiella papillae vary in number from none to three (Cohen 

 and Berrill, 1936). These authors admit the possibility of papillar recon- 

 stitution but regard it as more probable that a papillar primordium is 

 variously divided or brought entirely into the right or left blastomere by 

 slight variations in plane of first cleavage. They also consider it probable 

 that presumptive neural tissue may become general ectoderm and take 

 part in the covering of the half -larvae; and they find, as did Chabry and 

 Conklin, that a sensory primordium may appear in right or left half- 

 larvae, but offer various suggestions to account for this in terms of mosaic 

 development rather than reconstitution. 



Pieces of the unfertilized egg of Ascidiella can be fertilized and develop 

 (Dalcq, 1932a, b, c). Pieces resulting from meridional section may de- 

 velop half-embryos, complete symmetrical forms, or forms with more or 

 less complementary defects in the two halves. Dalcq, however, does not 

 regard even development of a whole larva from a half-egg as "true regula- 

 tion" but suggests that locahzation of cytoplasmic substances in the un- 

 fertilized egg gives it a bilaterally symmetrical structure and that merid- 

 ional section in the median plane divides these substances symmetrically, 

 so that all are present in each half and a whole larva develops from each. 

 However, what would normally give rise to half of the symmetrical pat- 

 tern gives rise in the half-eggs divided in the median plane to the whole 

 symmetry pattern. To maintain that reconstitution does not take place 

 in such cases seems scarcely in accord with the facts. Dalcq also finds 

 that small portions may be removed from apical or basal polar regions 

 without preventing normal development and that gastrulation occurs in 

 pieces down to one-fifteenth the egg volume; but in apical pieces there is 

 excess of ectoderm, in basal pieces ectodermal deficiency. 



These results indicate a considerable capacity for reconstitution in the 

 unfertiHzed ascidian egg. More recently Dalcq (1935, 1938^) has con- 

 cluded that it is not possible to establish for ascidians a topographic locah- 



