that _special_ 'primary constituents' are necessary, without which the relevant part cannot arise. The indispensableness of these primary constituents, the determinants, seems to me to depend on the fact that the new structure cannot be built up simply by procuring organic material, but _that specially hewn stones, different in every case, are necessary_, which can only be supplied in virtue of an historical transmission, or, to abandon the metaphor, because the vital units of which the organ is to be reconstructed possess a specific character and have a long history behind them; thus they can only arise from such vital units as have been handed on through generations, that is, from the determinants. But these primary constituents are given to the different forms of life in very varying degrees and in very unequal distribution, and as far as we can see according to their suitability to an end. LECTURE XXII SHARE OF THE PARENTS IN THE BUILDING UP OF THE OFFSPRING The ids are 'ancestral plasms'--The reducing division brings about a diversity of germ-plasm in the germ-cells--Bolles Lee's 'Neotaxis' even in the primordial germ-cells--Häcker's observations on the persistent distinctness of the maternal and paternal chromosomes--Identical twins--The individuality is determined at fertilization--Unequal share of the ids in the determination of the offspring--Preponderance of one parent in the composition of the offspring--Certain ids of the ancestors remain unchanged in the germ-plasm of the descendants--Struggle of the Biophors--Alternation of the hereditary sequences in the parts of the child--Reversion--Datura-hybrids--Zebra-striping in the horse--Three-toed horses--New experiments in hybridization among plants by Correns and De Vries--Xenia. As far as the phenomena of regeneration and budding are concerned, we have not been able to do much more than bring them under a formula, which harmonizes with the germ-plasm theory. But the case is different with the actual phenomena of inheritance in the restricted sense, for instance, with regard to the transmission of individual peculiarities _from parent to child_. Here the theory really increases our insight and lets us penetrate deeper into the causes of the phenomena; it is here no longer a mere 'portmanteau-theory.'