196 



INTERNAL FACTORS 



IV. i 



on the part of the egg from which it has been removed. In 

 the case of the colourless egg of EcJiinns it is only possible to 



guess at the nature of 

 the piece by observing 

 the mode of segmenta- 

 tion. 



The first and second 

 divisions may both be 

 equal, as also the third 

 and fourth ; in this case 

 (Driesch supposes) a 

 large fragment is in- 

 volved. Again, the first, 

 second, and third may 

 be normal, but the fourth 

 and fifth equal : no 

 micromeres, therefore, 

 are formed, and the 

 fragment is probably 

 derived from the animal 

 hemisphere. Or while 

 the first and second are 

 equal, one of the four 

 cells will divide un- 

 equally; in the next 



division a large or small 



FIG. 105. The potentialities of the cells & . 



of embryonic organs; . Normal Pluteus of number ot micromeres 

 Sphaerechinns ; 6. Pluteus reared from a frag- } s formed according 

 ment of a gaatrnla ; c, e. Normal Bipinnaria 



of Asteriasglacialis d, f. Bipinuaria from presumably, as a larger 

 the vegetative half of a gastrula ; g. Larva or smaller portion of the 

 of Asterias with typical tripartite gut, but . . , 



no coelom, from the vegetative half of a micromere region has 

 gastrula removed after development of the been included in the 

 coelom sacs. (After Driesch, 1896.) fragment (Fig. 106). 



In another case what appears to be a meridional half 

 divides equally twice, and then two of the four equally (meso- 

 meres), two unequally (macro- and micromeres) (Fig. 107), while 

 a supposed vegetative half is segmented to form four large and 

 four small cells. 



Thus the type of segmentation is determined by the initial 



