48o PATTERNS AND PROBLEMS OF DEVELOPMENT 



ganization. For organization a spatial pattern of some sort is essential, 

 and it is not at present evident how a sterol or other particular substance 

 can, of itself, originate an orderly and definite spatial pattern. The prob- 

 lem of organization is still the problem of the spatial pattern in the em- 

 bryo and any other developmental sytsem. In the amphibian the natural 

 inductor does not originate this pattern but is a part of it and plays a role 

 in modifying it. The foreign inductors merely modify it by local activation 

 or otherwise. The spatial pattern, whether we call it a gradient, a gradient 

 system, or something else, is the real organizer. In this connection Wad- 

 dington's recent discussion of organization (see p. 455, footnote 8) is of 

 particular interest because of his inability to throw any real light on 

 the problem. 



INDUCTION IN EARLY DEVELOPMENT OF OTHER CHORDATES 



ASCIDIANS 



The ascidian egg was earlier regarded as an extreme "mosaic," but re- 

 cent experiment has shown that a considerable capacity for reconstitution 

 is present (p. 577). Moreover, according to the most recent work with 

 isolated blastomeres, an inductor is concerned in early development. The 

 posterior blastomeres of the four-cell stage and the apical blastomeres of 

 the eight-cell stage are found to be incapable of independent differentia- 

 tion when isolated, but the anterior basal blastomeres of the eight-cell 

 stage can differentiate independently and are necessary for the differen- 

 tiation of other parts. These blastomeres represent presumptive noto- 

 chord, nerve cord, entoderm, and some mesoderm, but ascidian anterior 

 half-embryos differ from amphibian dorsal half-embryos in that they do 

 not reconstitute (Rose, 1939).-'' 



According to other experiments, removal of various parts of the ascidian 

 embryo some time before gastrulation gives no certain evidence of induc- 

 tion (von Ubisch, 1940, and another paper, 1940, Arch. Entw'mech., 140). 

 After removal of prospective notochord material the nervous system and 

 other organs are normal. After removal of all prospective muscle and 

 most of mesenchyme other organs develop without evidence of induction 

 by mesoderm. When prospective neural material is removed, no nervous 

 system develops; evidently there is no neural induction in other ectoderm 



3' Comparison with amphibian development raises the question whether the two sides of 

 the egg and embryo designated by ConkHn (1905a) and others as anterior and posterior are 

 not more nearly dorsal and ventral; neural plate and notochord develop from the "anterior" 

 basal cells of the eight-cell stage. 



