THE FORiMATIOX OF GERJI-CELI^ 205 



ectoderm or mesoderm in another : or. again, in a later stage, 

 only a rudiment of a particular part, organ, or portion of an or- 

 gan belonging to the germinal layer in question, may be present 

 in an individual cell. But if we inquire whether the entire body 

 could arise from each of these cells, known facts give a very 

 decided answer in the negative. Only one, or a few perfectly 

 definite cells amongst them, which we speak of as germ-cells. 

 can reproduce the whole animal under favourable circumstances. 

 This is true of all the higher Metazoa : the cells of the seg»ient- 

 hig ovion are completely dissimilar as regards their hereditary 

 value, although they are all 'young'' and 'embryonic,^ and are 

 not infrequently quite similar in appearance. It therefore 

 seems to me to follow from this, as a logical necessity, that the 

 hereditary substance of the egg-cell, which contains all the 

 hereditary tendencies of the species, does not transmit them /;/ 

 ioto to the segmentation-cells, but separates them into various 

 combinations, and transmits these in groups to the cells. I have 

 taken account of these facts in considering the regular distribu- 

 tion of the determinants of the germ-plasm and the conversion 

 of the latter into the idioplasm of the cells in the dilTerent stages 

 of ontogeny. All these cells contain • embryonic substance.' but 

 the determinants contained in one set dift'er from those in 

 another, and therefore contain different hereditary tendencies. 

 Hence it is comparatively meaningless to speak merely of an 

 ' embryonic substance.' 



De Vries regards some of my views in a very different way. 

 and from an entirely different aspect. In an extremely able 

 manner he brings forward a number of facts concerning hered- 

 ity in plants, and finds that they usually do not fit in with my 

 views. I have followed his deductions with great interest, 

 and have gratefully made use of the facts which he has brought 

 forward ; but I nevertheless believe that the chasm which sepa- 

 rates his views from mine can be bridged over. 



In the first place, de Vries accuses me of having taken a one- 

 sided view of the question by considering the processes as they 

 occur in animals only : in these it may be possible, as I have 

 as.sumed. to draw a sharp line of distinction between somatic- 

 and germ-cells, but this cannot be done in the case of plants. 

 In the latter, those series of cells which I have called germ- 

 tracks may give rise to many other cells besides germ-cells, 

 although this as a general rule is only exceptionally the case : 



