Experimental Studies on Germinal Localization. 257 



as thus stated the conclusion is somewhat misleading? I cannot 

 think otherwise. The fundamental conception that the develop- 

 ment of every part is conditioned by that of the organism as a 

 whole is one that every embryologist must accept; but it seems 

 to me that Driesch, whom no one will consider a partisan of the 

 mosaic-theory, expresses the truth when he says (Analytische 

 Theorie, p. 94), "In diesem Sinne ist nun Selbst-differenzieruno- 

 einmal angelegter Teile ein wesentliches Merkmal der Ontoge- 

 nese; ja sie ist in Hinsicht auf die spatere Einheitlichkiet und das 

 physiologische Zusammenwirken unaphangig entwickelter Gebilde 

 von einem ganz eigenartigen Interesse" (Italics original). The 

 fact must be recognized that the developmental energies and poten- 

 cies undergo a secondary distribution among the cells or tissues 

 at an earlier or later period, and in varying degrees, which in- 

 volves corresponding limitations in the secondary centers thus 

 created. We have long been familiar with such limitations in the 

 case of the "germ-layers," though the experimental evidence has 

 shown that they are here less rigid than was formerly supposed. 

 They have been experimentally shown with great clearness by 

 Driesch ('95) in the structures of the blastula, gastrula, and 

 young larva of the echinoderm at successive stages. The ex- 

 perimental results demonstrating the mosaic-character of cleav- 

 age have merely shown that similar restrictions of potency may 

 occur still earlier, so as to become manifest even in the early 

 cleavage-cells. Now, it is clear that the primary localization of 

 formative stuffs in the unsegmented egg is essentially an act of 

 the "organism as a whole;" and even though a complete preform- 

 ation and prelocalization of specific stuffs for every cell and tissue 

 were assumed — and I believe with Boveri and Fischel that such 

 an assumption is not necessary or even probable — we should not 

 escape the necessity for assuming such action of the whole. That 

 the egg undergoes a definite development during its ovarian his- 

 tory and in the stages preceding cleavage, we have evidence both 

 cytological and experimental. The character of the primary seg- 

 regation-pattern thus determined is indeed determined by the egg 

 as a whole, and the localization thus initiated forms the primary 

 basis of the specification of the blastomeres and organs that de- 



