Milwaukee-Downer College
TEN FIGURES
Few biological problems offer greater difficulties than those that attempt to deal concretely with the factors involved in the process of differentiation. In the light of our present knowledge, as ~7ell as on purely a priori grounds, the two extreme views, that of Driesch and his followers, that "the prospective value of a blastomere is a function of its position, " and the mosaic theory in its most pronounced form, that development is to be regarded as a mosaic work of self-differentiating cells,"2 seem equally untenable. The work of numerous investigators on a great variety of forms has established beyond question that when certain ooplasmic substances are destroyed the tissues into which they normally develop are lacking. On the other hand the fact that dcveloprnent may proceed for a time after the destruction of portions of the organism does not, for many reasons, demonstrate that the remaining portions are in any degree self-diff ereiitiating. First of all it is noticeable that development after the injury depends not so niuch on the extent as on the character of the substances destroyed, For example, Conklin,3 in the study of ascidian eggs, found that a right or left half enibyro develops xiiucti farther and much more normally than an anterior or posterior half, and from 1 I wish t o express my indebtedness to Professor F. R. Lillie, Director of the Wood's Hole Marine Biological Laboratory, and to Professor S. J. Holmes, of the University of Wisconsin, for many courtesies rcceivcd at the two institutions named while this paper was in preparation.