i86 NATURAL SCIENCE. Sept., 



while the various determinants are being marshalled to their places 

 may be called " heirs-unequal " division (erbungleiche Theilung), for 

 in each stage of it the daughter-cells receive different sets of deter- 

 minants. Thus, if, as Weismann supposes, the first cleavage plane 

 determines the longitudinal axis of the future body, the daughter-cell 

 on the right would receive all the determinants for the right side of the 

 body, the daughter-cell on the left all the determinants for the left side. 

 This process of heirs-unequal division would be the characteristic 

 method of nuclear division in all ontogenies, heirs-equal division 

 occurring only in the few cases necessary to separate off the sexual 

 cells. Later on in development, when the determinants have been 

 completely sorted out, heirs-equal division would re-establish itself, for 

 a determinate or independently variable part of the body generally 

 would consist of a large number of cells resulting from the simple 

 numerical multiplication of the cell or group of cells which, by heirs- 

 unequal division, had received the complete set of determinants for 

 the tissue in question. 



Now Hertwig strikes at the root of all this process, and there- 

 fore at the root of the whole Weismannic theory of heredity, by 

 saying that heirs-unequal division does not occur. Against it he 

 brings up five groups of facts. 



Groups of Facts against Heirs-Unequal Division. 



I. — Unicellular Organisms. 

 Among single-celled organisms, we know only of heirs-equal 

 division. It is the means by which the species are perpetuated. 

 Were the division a case of heirs-unequal, then there would result, 

 not daughter-organisms of the same species, but daughter-organisms 

 of new species, and all our evidence is against this happening. But 

 although the nucleus divides equally, it is not necessary that the 

 organisms of the new generation be at once like their parent. In 

 many protozoa an ontogeny is gone through. The results of division, 

 for instance, in the fixed Acinetan Podophrya gemmipara are free- 

 swimming ciliated forms, which, after a vagrant period, settle down 

 into the fixed adult form. Similarly, among Gregarines quite compli- 

 cated larval phases are gone through. None the less, in these and 

 similar cases, the exact adult form finally appears. From such 

 occurrences the inference is plain. Because cells are unlike we must 

 not conclude that their nuclear plasm is unlike. To use Weismann's 

 phraseology, some of their determinants may remain latent for a 

 time, and while the complete adult form will appear ultimately, in the 

 meantime the cell may present appearances very different. 



II. — Loiver Metazoa and Metaphyta. 



In the case of such organisms as the filamentous Algae the filament 



is, so far as can be judged, produced by heirs-equal division. Yet any 



of the resulting cells, similar to all appearance, may turn into sexual 



cells. If, with Weismann, it is assumed that the distinction between 



