No. 3.] AMPHIOXUS AND THE MOSAIC THEORY. 607 



while others become active and determine the specific form 

 and activities of the cell. Finally, the physiological specializa- 

 tion of the idioplasm is brought about by the interaction of the 

 cell with its fellows in the cell-complex, in accordance with the 

 principle: "Die Theile eines Organismus entwickeln sich in 

 Beziehung zu einander, oder die Entwicklung eines Theiles ist 

 abhangig von der Entwicklung des Ganzen " (13, p. 480). 

 Driesch is still more explicit (though he does not frame any 

 hypothesis regarding the idioplasm), and his views form the 

 opposite extreme to the mosaic theory. From his experiments 

 on sea-urchin eggs, Driesch concludes that the ultimate fate of 

 any particular blastomere is determined by its relative position 

 in the mass; that is, to quote his own striking aphorism, " ihre 

 prospective Bedeutung ist eine Funktion des Ortes " {cf. His. 

 No. 16, p. 120). This conclusion, essentially in agreement with 

 Hertwig's, rests upon a series of experiments so ingenious in 

 conception, and brilliant in results, as to stand beside those of 

 Roux as the mark of a new era in the history of embryology. 

 The experiments on the alteration of cleavage-forms by pres- 

 sure, temperature and other agents, taken in connection with 

 the phenomena exhibited by isolated, or partially separated 

 blastomeres, show that the embryo develops as a whole, as a 

 unit, and demonstrate the truth of the principle urged by 

 Whitman, Hertwig and others, that "the organism, as a whole, 

 controls the formative processes going on in each part " (Whit- 

 man, 37, p. 48). I shall endeavor to interpret the leading 

 facts in the development of Amphioxiis, and some other forms, 

 in accordance with this principle, and for the sake of drawing 

 the issue clearly shall adopt the idioplasm hypothesis, leaving 

 aside, however, as an open question, the problem of the exact 

 seat of the idioplasm and its mode of specialization. 



There is no escape from the conclusion that in Amphioxns 

 as in Echinus, the prospective value of the blastomeres of the 

 2-celled or 4-celled stage is a "function of the location." It 

 is a statement not of theory but of fact that the mode of 

 development of each blastomere in these stages is determined 

 by its relation to its fellows, for the mode of development 



