.52 



CHEMODIFFERENTIATION 



a liumbc. of "micromere quartets" are formed, surrounding 

 !iie aiiimal pole in a spiral arrangement (so-called ''spiral 

 cleavage''). At first, the pole-plasm is situated in one of the 

 first four blastomeres. Later, it is distributed over two of the 

 micromeres produced by this cell; these micromeres are larger 

 than the others. They are called the first and second somato- 

 hlasts (Fig. 17b). In later development, the first somatoblast 

 soon divides into a right and a left half. Each of these halves 



Fig. 18. Eggs of Tubifex. a: normal ^gg, with mitotic spindle (sp), 

 and animal {an) and vegetative {veg) pole plasm; b: centrifuged 

 egg (centrifugal force parallel to the axis of the egg), strong 

 accumulation of pole-plasm (p.p.) at the centripetal pole (above), 

 whereas only little pole-plasm is formed at the centrifugal pole; 

 c: egg centrifuged with the centrifugal force acting at right angles 

 to the egg axis, animal pole (an) at one side; d: as c, somewhat 

 later, centripetal pole above; the pole-plasm has accumulated at the 

 place of the original animal and vegetative poles. After Lehmann. 



