THE DEVELOPMENT OF ORGANISMS 757 



The annual succession of foliage on perennial plants is, of course, 

 not a case of special regeneration, but a normal and seasonal replace- 

 ment, due to the buds usually developed in the axils of the full- 

 grown leaves. Yet even here we have all seen renewal of buds after 

 a late spring frost or blight, and the like can be provoked by the 

 propagator. Similarly one must not too sharply separate from the 

 ordinary growth-process what one often sees on an injured tree- 

 stem — the mending of the wound by the slow outgrowth from 

 the cambium layer over the bared wood-surface. 



The general impression suggested by the work of "Natura Medica- 

 trix" among plants is that more or less regenerative capacity is 

 almost universal, and may be called forth in exigency, or stimulated 

 by conditions of vegetative exuberance. The frequency of regeneration 

 in plants as compared with animals must be associated with the 

 relatively simple differentiation of most structures, in lower forms 

 especially. In higher forms it is associated with the persistence 

 of large tracts of embryonic or rejuvenescent tissue, such as the 

 cambium layer so familiar to us in tree-stems. 



From a detached morphological point of view, the zoologist 

 may claim that much as he might roughly imitate most orders of 

 Flowering Plants by varying the disposition of the tentacles in 

 Coelenterate polyps, which often recall the plant's axis and append- 

 ages, so he may roughly compare the general structural level of 

 plants with that of the Coelentera among animals. In both groups 

 the regenerative capacity is almost unlimited ; and in both the phe- 

 nomenon of alternation of generations finds its finest expression. 



INTERPRETATION.— It used to be believed in the early days of 

 experimental embryology that the divisions of the egg-cell, and of 

 the resulting segmentation cells, were qualitative. That is to say, 

 there is a parcelling out and segregation of particular hereditary 

 characteristics, some to one group of cells and some to another, the 

 result being differentiation. This view made it very difficult to 

 understand those cases of re-growth and regeneration where a 

 separated piece gives rise to a complete whole, or an undifferentiated 

 stump gives rise to a complex organ, such as the eye at the tip of 

 a snail's horn. But the idea of qualitative division in ordinary 

 development was erroneous. The nuclei are divided and redivided 

 with meticulous accuracy, a longitudinal half of each chromosome 

 passing to each daughter cell. According to the stimulation that each 

 cell receives from its neighbours, and from elsewhere (e.g. by hor- 

 mones), and from the environment, there is an evoking of the 

 formative activities of different genes or factors in the chromosomes 

 carried by each and every ceU. Thus what evokes an activation of 

 this or that hereditary initiative or gene is the precondition of 

 regeneration ; and the line of investigation is to discover what these 



