NATURE AND CAUSES OF DIFFERENTIATION 



419 



plete embryo, and gives rise to a defective larva, having only four, 

 two, or one row of swimming-plates (Fig. 189); and Fischel's obser- 

 vations give strong reason to believe that each of the eight micromeres 

 of the sixteen-cell stage is definitely specified for the formation of one 

 of the rows of plates. In like manner Crampton ('96) found that in 

 case of the marine gasteropod Ilyanassa isolated blastomeres of two- 

 cell or four-cell stages segmented exactly as if forming part of an 

 entire embryo, and gave rise to fragments of a larva, not to complete 

 dwarfs, as in the echinoderm (Fig. 190). Further, in embryos from 

 which the " yolk-lobe " (a region of that macromere from which the 

 primary mesoblast normally arises) had been removed, no mesoblast- 

 bands were formed. Most interesting of all, Driesch and Morgan 

 discovered that if a part of the cytoplasm of an unsegmented cteno- 

 phore-egg were removed, the remainder gave rise to an incomplete 

 larva, showing definite defects (Fig. 189, E, F). 



In none of these cases is the embryo able to complete itself, though 

 it should be remarked that neither in the ctenophore nor in the snail 

 is the partial embryo identical with a fragment of a whole embryo, 

 since the micromeres finally enclose the macromeres, leaving no sur- 

 face of fracture. This extreme is, however, connected by a series of 

 forms with such cases as those of Amphioxus or the medusa, where 

 the fragment develops nearly or quite as if it were a whole. In the 

 tunicates the researches of Chabry ('87), Driesch ('94), and Crampton 

 ('97) show that an isolated blastomere of the two-cell stage undergoes 

 a typical half -cleavage (Crampton), but finally gives rise to a nearly 

 perfect tadpole larva lacking only one of the asymmetrically placed 

 sense-organs (Driesch). Next in the series may be placed the frog, 

 where, as Roux, Endres, and Walter have shown, a blastomere of the 

 two-cell stage may give rise to a typical half-morula, half-gastrula, 

 and half-embryo 1 (Fig. 182), yet finally produces a perfect larva. A 

 further stage is given by the echinoderm-egg, which, as Driesch 

 showed, undergoes a half-cleavage and produces a half-blastula, which, 

 however, closes to form a whole before the gastrula-stage (Fig. 183). 

 Perfectly formed though dwarf larvae result. Finally, we reach Amphi- 

 oxus and the hydromasae in which a perfect "whole development" 

 usually takes place from the beginning, though it is a very interest- 

 ing fact that the isolated blastomeres of Amphioxus sometimes show, 

 in the early stages of cleavage, peculiarities of development that recall 

 their behaviour when forming part of an entire embryo. 2 



We see throughout this series an effort, as it were, on the part of 

 the isolated blastomere to assume the mode of development character- 

 istic of a complete egg, but one that is striving against conditions that 



1 This is not invariably the case, as described beyond. 



2 Cf. Wilson, '93, pp. 590, 608. 



