IV. i INITIAL STRUCTURE OF THE GERM 197 



structure, including- the physical constitution, of the ovum alone, 

 a conclusion in which Boveri concurs. 



All these fragments will give rise to normal larvae, provided 

 they are not too small. The least egg- fragment that will gastru- 

 late has the same germinal value as the least blastomere, namely, 



FIG. 106. Segmentation of an egg-fragment of Echinus supposed to 

 contain the whole of the inicromere area of the egg, and some of the 

 animal hemisphere, a. Eight cells, three equal pairs and one unequal 

 pair ; b. sixteen cells, four of the eight having formed microineres, the 

 other four divided equally. (After Driesch, 1896.) 



/^ (calculated from the volume of the smallest gastrulae found). 

 Morgan, however, estimates the smallest egg-fragment capable of 



giving rise to a normal larva at from ^ to 

 mere that will gastrulate only ^. 



the least blasto- 



FIG. 107. -Segmentation of an egg-fragment (Echinus) supposed to 

 be an exact meridional half. a. Eight cells, four being mesomeres, 

 two macromeres and two microineres ; b. sixteen cells. (After Driesch, 

 1896.) 



In partial blastulae, gastrulae, or larvae the number of cells is 

 proportional to the germinal value ; and this is true not merely 

 of the whole embryo or larva, but of each of its organs, the 



