CHEMODIFFERENTIATION 



55 



In the eggs of many worms and molluscs, a special pole-plasm 

 is accumulated at the vegetative pole. This shows a character- 

 istic behaviour during the first few cleavages. As soon as the 

 first cleavage furrows the egg, the pole-plasm becomes to some 

 extent separated from the rest of the egg, and forms the polar 

 lobe. This remains connected with one of the two blastomeres, 

 and later fuses with it so that all the pole-plasm is taken up 

 by one cell. This process may be repeated several times in the 



O 



Fig. 20. Cleavage in Dentalium. (a) uncleaved egg, with animal 

 pole-plasm and polar bodies at the animal pole, and vegetative 

 pole-plasm at the vegetative pole; (b) first cleavage, the vegetative 

 pole-plasm, now concentrated in the polar lobe, almost completely 

 separated from the rest of the egg; (c) 2-cell stage, the polar lobe 

 has fused with one of the cleavage cells; (d) second cleavage, polar 

 lobe has been formed anew; (e) 4-cell stage, polar lobe fused with 

 one of the cells; (/) third cleavage, polar lobe formed once more; 

 ig) 8-cell stage, pole-plasm in one of the cleavage cells. After Wilson. 



