210 Edwin G. Cor^klin. 



Experiment confirms, therefore, what observation of the normal 

 development plainly indicates that these strikingly different ooplas- 

 mic substances are not totipotent, but that as early as the close of the 

 first cleavage arid probably much earlier, they are differentiated for 

 particular ends, and that if they develop at all they give rise to 

 organs of a particular kind. These materials are, therefore, " organ- 

 forming substances" and the areas of the egg in which they are 

 localized are ^^ organ- forming regions." 



I need not here point out the similarity between this conclusion 

 and the well-known theories of Sachs and His, nor the differences 

 between my results and the commonly accepted view that the egg 

 is composed of "simple undifferentiated protoplasm" or that 

 "cleavage is a mere sundering of homogeneous materials capable 

 of any fate," or that "the prospective value of a blastomere is a 

 function of its position." Whatever may be true of other animals 

 these things are certainly not true of ascidians. 



While there are few, if any, other cases known in which the 

 differentiations of the ooplasm are so striking or so numerous as in 

 the egg of Cynthia there can be no doubt that organ-forming sub- 

 stances are present in the eggs of many animals. In particular 

 the works of Fischel ('97, '98, '03) on the Ctenophore, of Boveri 

 ('01) on Strongylocentrotus; of Wilson ('04) on Dentalium and 

 Patella and of Conklin ('03) on Physa, Planorbis and Limnaea 

 have shown that distinct kinds of protoplasm are present in these 

 eggs which are destined in the course of development to give rise 

 to particular germ layers or organs. In the light of these discov- 

 eries it can scarcely be doubted that the general cause of mosaic 

 development is to be found in the presence in the egg or blasto- 

 meres of distinct kinds of protoplasm, or of organ-forming 

 substances. 



2. Localization of Ooplasmic Substances. 



The three principal substances in the egg of Cynthia, viz: the 

 clear, the yellow and the gray, are already present and localized 

 * in the oocyte before it escapes from the ovary. The yellow 

 (mesoplasm) forms a peripheral layer around the entire egg; the 

 clear (ectoplasm) is the clear achromatic substance within the 

 germinal vesicle; the gray (endoplasm) constitutes most of the 



