PONFICK: _Experimentelle Beaträge zur Lehre von der Transfusion_. _Virchow's Archiv._, vol. lxii. BERESOWSEY: _Ueber die histologischen Vorgänge bei der Transplantation von Hautstücken auf Thiere einer anderen Species_. _Ziegler's Beiträge zur pathologischen Anatomie und zur allgemeinen Pathologie_; Jena, 1893. PART II. THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT OF ORGANISMS.[17] Now that criticism of the germplasm theory has given us a bias in the right direction, it is necessary to map out more clearly the path along which solution of the problem may be sought. In general terms, our problem is the necessary origin from an egg, always of the same organism, with its manifold characters, and the explanation must avoid the attribution to the egg of characters foreign to its nature as a cell. This is the more necessary as Weismann objects to the supposition that cell-division is doubling, holding that the supposition allows neither an explanation, nor even the beginning of an explanation, of the differences that arise among cells while the differentiation of the body occurs. 'Any explanation must in the first place account for this differentiation,' says Weismann (_Germplasm_, p. 224); 'that is to say, the diversity which always exists amongst these cells and groups of cells arising from the ovum must be referred to some definite principle. In fact, no one could even look at it as giving a partial solution of the problem, if differentiation is supposed to be due to that part alone of the germplasm becoming active which is required for the production of the cell or organ under consideration. But the higher we ascend in the organic world, the more limited does the power of producing the whole from separate cells become, and the more do the numerous and varied differentiations of the soma claim our attention and require an explanation in the first instance. The presence of idioplasm in all parts containing the primary constituents does not help us in this respect.' With this I cannot agree. Naturally, Naegeli, De Vries, Driesch and I assume that, of the many rudiments present in every cell, only some come to activity in each special case, and that the selection of those that become active is due to causes arising in the course of development. Our conception of the nature of these causes, and of their place of origin, is diametrically opposed to Weismann's. Weismann would make the causes of this orderly development of the rudiments reside in the germplasm itself; for he considers that to be not only the material but the motive force of the course of development. According to him, every cell _must_ have become what it is, because it was provided only with the definite rudiments assigned it beforehand, according to the plan of the development of the germplasm.