56 



CHEMODIFFERENTIATION 



subsequent cleavages (Fig. 20). Here again the entire pole- 

 plasm finally finds its way into the first and second somato- 

 blasts. Wilson (1904) has shown experimentally that, in these 

 cases too, the pole-plasm is of great importance for further 

 development. Complete removal of the polar lobe results in 

 defective embryos, lacking certain organs ; partial removal leads 

 to poor development of these organs (Fig. 21d-f). Wilson 

 isolated parts of the eggs at later stages as well. In such cases 

 the cells that contained the pole-plasm differentiated into small 

 larvae, which were complete, though not harmoniously built, 



Fig. 21. Dentalium. (a) normal trochophore larva; (h) larva from 

 an isolated blastomere without pole-plasm; (c) larva from a cleavage 

 cell with pole-plasm; (d) larva from an egg, the first polar lobe of 

 which had been removed; (e) as d, but only half of the first polar 

 lobe resected; (/) larva developing from an egg after resection of 

 the second polar lobe. After Wilson. 



