No. 3-] AMPHIOXUS AND THE MOSAIC THEORY. 615 



further qualification, that the mosaic-like character of the 

 ontogeny emerges from the indifferent condition of the early 

 stages at different periods in different animals, and in many 

 cases appears more or less distinctly from the beginning. 

 We are thus enabled, in a measure, to reconcile the appar- 

 ently conflicting results of Roux on the one hand, and those 

 of Hertwig and Driesch on the other. It is true that no mid- 

 dle ground is possible in the question of qualitative versus 

 quantitative division ; but it is otherwise with the external 

 phenomena of cleavage. I have endeavored to show that the 

 phenomena of regeneration are not incompatible with a modi- 

 fied form of the mosaic theory, in which the hypothesis of 

 qualitative division is repudiated. Thus modified, the mosaic 

 theory is of the utmost importance, and is destined, I believe, 

 to form the basis of all exact and thorough investigations on 

 animal ontogeny. 



Biological Laboratory of Columbia College, 

 New York, April 10, 1893. 



