582 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the mosaic condition is more nearly approached. Removal of various parts 

 of the embryo preceding gastrulation affords no certain evidence of re- 

 constitution, except possibly of entoderm, or of induction (von Ubisch, 

 1940). 



CLEAVAGE AND PATTERN IN AmpMoXUS 



Development of Amphioxus shows similarities in general regional pat- 

 tern to ascidian and amphibian development. The apical hemisphere is 

 chiefly ectodermal, the basal region entodermal; a mesodermal crescent 

 lies posteroventrally, and opposite this is the area from which notochord 

 and neural plate develop." In the separation of mesodermal and chordal 

 areas this pattern resembles the ascidian and differs from the amphibian 

 pattern. Evidence of dorsiventrality appears in the undivided egg, and 

 the first cleavage plane usually coincides with the median plane; but 

 cleavage is, in general, less distinctly determinate than in ascicUans and, 

 except for differences resulting from the smaller amount of yolk, gastrula- 

 tion resembles amphibian gastrulation. 



Development of isolated blastomeres led E. B. Wilson (1893) to the 

 conclusion that Amphioxus development is not mosaic. Conkhn (1933) 

 agrees with Wilson that whole normal larvae may develop from 1/2 

 blastomeres but maintains that this is possible only if the blastomeres are 

 right and left halves. Partly separated blastomeres of the two-cell stage 

 show various degrees of twinning, the two partially united forms appar- 

 ently developing independently of each other, often with polar axes in 

 different directions in consequence of blastomere rotation. Partial separa- 

 tions at the four-cell stage develop into three or four blastulae or gastrulae, 

 but further development is rare and abnormal. Conkhn regards Amphi- 

 oxus development as essentially mosaic, except for reconstitution of 1/2 

 blastomeres into whole individuals. However, in the hght of the ascidian 

 experiments, it seems possible that further experiment may lead to some- 

 what different conclusions: an inductor region may be concerned in Am- 

 phioxus development, and transplantations and combinations of blasto- 

 meres may bring to light relations of parts not shown by isolations. » 



CLEAVAGE AND DEVELOPMENTAL PATTERN IN OTHER FORMS 



In many animals cleavage pattern is less definitely determinate than 

 the patterns discussed or apparently entirely or almost entirely indeter- 

 minate. In forms with total cleavage (holoblastic) the planes of early divi- 



" Conklin, 1932. References to earlier literature given there. 



