CLEAVAGE AND DEVELOPMENTAL PATTERN 581 



which normally produce little ectoderm, give rise to forms completely 

 covered with ectoderm. Level of gastrulation is not predetermined; and 

 Tung suggests a double polar gradient, animal and vegetal, such as 

 Runnstrom has postulated for the sea urchin, and a quantitative deter- 

 mination of ectoderm and entoderm in relation to gradient-level. Here, 

 as in the sea urchin, the vegetal gradient may be secondary, at least as 

 far as dynamic factors are concerned. 



According to Tung, the micromeres give rise to cerebral vesicle, and 

 macromeres to neural cord in many cases; and presence of notochord is 

 not necessary for formation of a cerebral vesicle, but the possibility of 

 induction by entoblast is not excluded. Presumptive neural tissue may 

 develop as general ectoderm, and supernumerary cerebral vesicles may 

 develop. Presence, absence, and variable numbers of pigmented cells in- 

 dicate that they, too, are not fixedly predetermined. The adhesive papil- 

 lae also are apparently determined in relation to other parts. 



Results of fusions, killing, and isolation of blastomeres and blastomere 

 groups have led von Ubisch to conclude that the general organ-forming 

 regions of the embryo, except ectodermal, are determined at the sixteen- 

 cell stage. Presumptive neural ectoderm may become epithelium, and 

 presumptive epithelium may become entoderm. However, the embryonic 

 regions are not sharply separated from each other but are gradients which 

 merge into each other {ineinander iihergehen) at their boundaries; these 

 boundary regions may develop into organs of either of the gradient systems 

 concerned. Within the general organ-forming regions determination is 

 not fixed at the sixteen-cell stage, but extensive reconstitutions {Regiila- 

 tionen) are possible.^' 



In short, it seems to be evident that ascidian development, often re- 

 garded as the most extreme case of mosaic development, is actually far 

 from a mosaic in early stages. As in various other eggs and embryos, cer- 

 tain parts are more stable than others and appear thus far to be definitely 

 restricted in potency; but the possibiUty remains that with other experi- 

 mental alterations of environmental relations of these parts other poten- 

 cies may be brought to light. Moreover, it appears that neither deter- 

 minate cleavage nor partial development of isolated parts provides an 

 adequate basis for the conclusion that ascidian development is, from the 

 beginning, a mosaic of independent parts. As development progresses, 



2' Von Ubisch, 19386, 1940, also 1939, "tjber die Entwicklung Ascidienlarven nach friih- 

 zeitiger Entfernung der einzelnen organbildenden Keimbezirke," Arch. Entiv'mcch., 139; and 

 alaterpaper, 1940, "Regulation und Determination im Ascidienkeim," ibid., 140. See also the 

 evidence for presence of an inductor region in ascidians on p. 480. 



