CHEMODIFFERENTIATIOX 



51 



tion of the protoplasmic material had been profoundly modified, 

 the pole-plasm still assembles under the cortex at the two 

 original poles of the egg (Fig. 18). The physico-chemical 

 composition of the pole-plasm is slightly different from that 

 of the rest of the protoplasm. For one thing, it is very rich in 

 certain enzymes (oxidases; Lehmann, 1948). During further 

 development, the two masses of pole-plasm unite in the centre 

 of the egg. In Tubifex, as in most worms and molluscs, cleavage 

 has a characteristic very regular pattern. First the egg divides 

 into four blastomeres. At the animal side each of these then 

 buds off a number of small cells, the micromeres. In this way 



'/Js/r 



Fig. 17. Development of Tubifex. (a) uncleaved egg, (an r=: animal, 

 veg =r vegetative pole-plasm); (h) 22-cell stage, first and second 

 somatoblast have been formed; (c) formation of the mesodermal 

 germ bands (nies.k.str.) from the primary mesoderm cells 

 ioermes.c), (tel = teloblasts); (d) older embryo, from the right 

 side, with right ectodermal germ band (ect.k.str.), and, at the hind 

 end of the latter, the teloblasts (tel.). After Penners. 



