4IO INHERITANCE AND DEVELOPMENT 



eggs separated by shaking to pieces the two-cell and four-cell stages. 

 Blastomeres thus isolated segment as if still forming part of an entire 

 larva, and give rise to a half- (or quarter-) blastula (Fig. 183). The 

 opening soon closes, however, to form a small complete blastula, and 

 the resulting gastrula and Pluteus larva is a perfectly formed dwarf 

 of only half (or quarter) the normal size. Incompletely separated 

 blastomeres give rise to double embryos like the Siamese twins. 

 Shortly afterward the writer obtained similar results in the case of 

 Amphioxus, but here the isolated blastomere behaves from the begin- 

 ning like a complete ovum of half the usual size, and gives rise to a 

 complete blastula, gastrula, and larva. Complete embryos have also 

 been obtained from a single blastomere in the teleost Fundulus 

 (Morgan, '95, 2), in Triton (Herlitzka, '95), and in a number of 

 hydromedusae (Zoja, '95, Bunting, '99); and nearly complete em- 

 bryos in the tunicates Ascidiella (Chabry, '87), Phallnsia (Driesch, 

 '94), and Molgula (Crampton, '98). 1 Perhaps the most striking of 

 these cases is that of the hydroid Clytia, in which Zoja was able to 

 obtain perfect embryos, not only from, the blastomeres of the two- 

 cell and four-cell stages, but from eight-cell and even from sixteen- 

 cell stages, the dwarfs in the last case being but one-sixteenth the 

 normal size. 



These experiments render highly improbable the hypothesis of 

 qualitative division in its strict form, for they demonstrate that the 

 earlier cleavages, at least, do not in these cases sunder fundamentally 

 different materials, either nuclear or cytoplasmic, but only split the 

 egg up into a number of parts, each of which is capable of producing 

 an entire body of diminished size, and hence must contain all of the 

 material essential to complete development. Both Roux and Weis- 

 mann endeavour to meet this adverse evidence with the assumption 

 of a " reserve idioplasm," containing all of the elements of the germ- 

 plasm which is in these cases distributed equally to all the cells in 

 addition to the specific chromatin conveyed to them by qualitative 

 division. This subsidiary hypothesis renders the principal one (i.e. 

 that of qualitative division) superfluous, and brings us back to the 

 same problems that arise when the assumption of qualitative division 

 is discarded. 



The theory of qualitative nuclear division has been practically dis- 

 proved in another way by Driesch, through the pressure-experiments 

 already mentioned at page 375. Following the earlier experiments of 

 Pfluger ('84) and Roux ('85) on the frog's egg, Driesch subjected 

 segmenting eggs of the sea-urchin to pressure, and thus obtained flat 

 plates of cells in which the arrangement of the nuclei differed totally 



1 The " partial " development in the earlier stages of some of these forms is considered 

 at page 419. 



