58o PATTERNS AND PROBLEMS OF DEVELOPMENT 



zation of hypothetical cytoplasmic substances agreeing with the data of 

 observation, that the occurrence of regulation in the ascidian egg must be 

 admitted, and that ascidian development, Hke that of other forms, must 

 be interpreted in dynamic terms. He regards the organization of the ascid- 

 ian egg as consisting of a polar gradient, the mesoplasm (Conklin's yellow 

 crescent), and a cortical, dorsiventral field with greatest concentration in 

 the region of the equator. These are regarded as dynamic factors and 

 morphogenesis is conceived as resulting from their interaction. 



Eggs from which extrusion of cytoplasm has been brought about by 

 puncture of the chorion and pressure may give rise to more or less normal 

 larvae (Reverberi, 1931), but what region of the cytoplasm is removed 

 in these experiments is not known. Isolation of the four apical and the 

 four basal blastomeres of the eight-cell stage results in alteration of later 

 cleavage in the former but not in the latter, and the conclusion is drawn 

 that the factors controlhng bilaterality are locahzed in the basal region 

 (Reverberi, 1933). Pieces of the unfertilized egg of Ciona, when fertihzed, 

 cleave like the whole egg, irrespective of the region which they represent 

 (Reverberi, 1936). Pieces of fertilized eggs with known orientation of 

 plane of section give chfferent results. Pieces above a certain size, obtained 

 by section parallel to the polar axis, cleave like whole eggs. Pieces ob- 

 tained by section vertical to the polar axis cleave like wholes if they con- 

 tain a zone localized in three-fourths of the basal hemisphere ; but pieces 

 of the apical hemisphere, or of this plus the adjoining basal fourth, cleave 

 radially. These experiments also indicate that factors determining bilat- 

 erality are localized in the basal region (Reverberi, 1937). 



These isolation experiments show that developmental potencies of at 

 least some regions of ascidian eggs are less strictly limited than most ear- 

 lier investigators believed. Experiments on translocation and combination 

 of blastomeres of Ascidiella aspersa provide further evidence that this is 

 the case (Tung, 1934). These experiments include rotations of the four 

 apical blastomeres (micromeres) of the eight-cell stage 90°, 160°, and 180° 

 on the four basal blastomeres, superposition of two groups of micromeres 

 with polar axes unchanged but with various degrees of rotation about this 

 axis, superposition of two two-cell stages with median planes at right 

 angles, and various isolation experiments. The only parts which appear 

 truly stable in Tung's experiments are the primordia of notochord, myo- 

 blasts, and probably of mesenchyme. Presumptive ectoderm and ento- 

 derm are "relatively equipotential." The four apical micromeres, nor- 

 mally ectodermal, can develop as entoderm; and the basal macromeres, 



