2l6 THE MOSAIC STAGE OF DIFFERENTIATION 



It will be remembered that in Dentalium (p. no), chemo-difFer- 

 entiated substances are present in the polar lobe and become in- 

 corporated in blastomere Z), with the result that of the cells of the 

 4-cell stage, only blastomere D is able to produce a complete larva, 

 but that the structures to which the polar lobe gives rise (apical 

 organ and post-trochal region with mesoderm) are full-sized, and 

 therefore disproportionately large. 



On the other hand, in Tubifex (p. 113), chemo-differentiated 

 substances are present in the pole-plasms, and likewise become 

 incorporated in blastomere Z), which, alone of the blastomeres of 

 the 4-cell stage, is capable of giving rise to a complete larva. But 

 this larva produced from D in Tiihifex is properly proportioned. 



Blastomeres AB, A, B^ or C of Dentalium and Tuhifex are in- 

 capable of giving rise to a complete larva, not because of any 

 positive determination to differentiate along the lines of their pro- 

 spective fates, but because of the negative fact that they lack the 

 essential ingredients for forming structures to which they do not 

 give rise in normal development. Further, it is clear that in Den- 

 talitim, regulation in blastomere D occurs in some respects but not 

 in others. The larvae thus formed have regulated as regards their 

 external form and show no trace of asymmetry, but the charac- 

 teristics dependent upon the polar lobe (apical organ and post- 

 trochal region) are disproportionately large. In Tubifex, on the 

 other hand, regulation in blastomere D seems to be complete. If 

 we are to make a conjecture as to the meaning of this distinction, 

 it wouM be that chemo- differentiation is more precocious in 

 Dentalium and results in a complete determination, quantitative as 

 well as qualitative, of the organ-forming substances contained in 

 the polar lobe. The state of affairs in Tubifex, on the other hand, is 

 more like that of the early half-gastrula of Triton, in which quanti- 

 tative regulation of the neural folds is still possible (see p. 239).^ 



The most striking demonstration of the presence of organ- 

 forming substances is that of the Ascidians, already referred to 

 (pp. 119, 123). The fertilised egg possesses a yellow crescent and 



^ In the absence of experiments involving the removal of the pole-plasms of 

 Tubifex, comparable to those in which the polar lobe of Dentaliwn is cut off, it is 

 impossible to rule out the suggestion made by Morgan (1927, p. 379) that the 

 pole-plasms of Tuhifex may be indices of some underlying peculiarity of organi- 

 sation, rather than organ-forming substances. 



