REGENERATION IN EGG AND EMBRYO 233 



small, also shows that the lack of power to develop, found in some 

 of the one-fourth and in many of the one-eighth blastulae, is not 

 the result of any special differentiation that they have undergone 

 during the cleavage period, but is due to their size. 



A recent series of experiments by Driesch (1900) on the develop- 

 ment of isolated blastomeres of the sea-urchin's egg has given more 

 exact data in regard to their limit of power to produce embryos, and 

 has shown the possibilities in these respects of different parts of the 

 egg. By means of a method discovered by Herbst (1900) it is pos- 

 sible to obtain isolated blastomeres more readily than by the some- 

 what crude shaking process. If the eggs, after fertilization and after 

 the removal of the membrane by shaking, are placed in an artificial 

 sea water, from which all calcium salts have been left out, the eggs 

 divide normally, but the blastomeres are not held firmly together, and 

 readily fall apart if the egg is disturbed. By means of a fine pipette 

 any desired blastomere or group of blastomeres can be picked out. 

 If these are returned to sea water they continue to develop. 



Driesch found that the one-half and one-fourth blastomeres 

 develop into proportionate gastrulae and larvae ; that the one-eighth 

 blastomeres, both of the animal and the vegetative hemispheres, some- 

 times produce gastrulae, and even the beginning of the larval stage 

 with the rudiments of a skeleton. There are certain differences 

 between the one-eighth larvae that come from the animal hemisphere 

 and those from the vegetative half. More of the one-eighth 

 blastomeres from the animal part of the egg die than from the 

 opposite part, but of those that remain alive a larger percentage 

 reach the gastrula stage than in the case of those from the vege- 

 tative pole ; their protoplasm moreover is not so clear as is that of 

 the larvae from the other hemisphere. These "animal pole" blasto- 

 meres develop faster than those of the other sort. The gastrulae 

 from the one-eighth blastomeres of the vegetative hemisphere do 

 not die so often after separation, the protoplasm of the larvae is 

 clearer, and they often produce long-lived blastulae with long cilia. 

 The blastulae often develop into gastrulae without mesenchyme. 

 These results show that although whole larvae may be produced from 

 the one-eighth blastomeres of both hemispheres, yet there are certain 

 characteristics that may be referred with great probability to differences 

 that are present in the protoplasm of the two hemispheres of the egg. 

 The differences are not in all cases sufficient to interfere with the 

 production of all the characteristic structures of the embryo, yet 

 traces of the origin of the larvae can be found in their structure. It 

 is probable that the so-called animal (or micromere) pole corresponds 

 to that part of the egg from which the archenteron is produced. 

 Hence the one-eighth blastulae from this hemisphere gastrulate 



