. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . FIG. 158.—Lithium larva of the seaurchin, Sphccrechinus granularis:A, Elongated blastula; B, evagi-nated gastrula. (After Herbst.) FACTORS IN ONTOGENY 279 the first cleavage of Synapta, and the cell lineage studies of zurStrassen on the eggs of Ascaris in which it was shown that adefinite status of outcome for each blastomere was determinedafter successive early cleavages. All these results seem to be A. FIG. 159.—A, Normal gastrula of sea urchin, Echinus
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . FIG. 158.—Lithium larva of the seaurchin, Sphccrechinus granularis:A, Elongated blastula; B, evagi-nated gastrula. (After Herbst.) FACTORS IN ONTOGENY 279 the first cleavage of Synapta, and the cell lineage studies of zurStrassen on the eggs of Ascaris in which it was shown that adefinite status of outcome for each blastomere was determinedafter successive early cleavages. All these results seem to be A. FIG. 159.—A, Normal gastrula of sea urchin, Echinus microtuberculatus; B, gastrula ofsea urchin, Sphcer echinus granularis, from a lithium culture. (After Herbst.) good evidence for preformation, that is, for a predeterminationof the role each part of the egg cell is to play in , Wilson is convinced that an obvious structural differ-entiation (bands, zones, delimited regions) can be seen in theundeveloped eggs of numerous animals, a differentiation corre-sponding to structural di-vergence in development. On the other hand, nu-merous results of experi-ment speak just as loudlyagainst preformation or pre-determination. Such areHerlitzkas half-sized Tritonembryos from the two sepa-rated first cleavage cells, Drieschs two half-sized and four quarter-sized sea-urchin pluteifrom the cells of the first and second cleavages, respectively,his eight and sixteen small gastrula1, and thirty-two tiny blas-tulse from the separate blastomeres of the third, fourth, andfifth cleava
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