548 Prof. G. Klebs. Alterations in Development and [June 25, 



external factors only supply material and energy to build up the organism, 

 without influencing the course of its development. But if this presumption 

 were really true, we should have to give up hope of attaining anything more 

 than purel)'' descriptive treatment of form. Fortunately, however, as modern 

 biology shows, we can prove more and more that development can be altered 

 in a far-reaching degree. The fact that the internal alterations causing the 

 definite form can be modified by alterations of environment is the key which 

 opens the door to research. 



It has been known for a long time that water-plants, when brought into a 

 relatively dry atmosphere, change their form, and numerous other examples 

 of form-modification due to environment are familiar to us. But now we 

 wish to elucidate the whole phenomenon of development, and to recognise 

 the conditions of every stage. Our aim is to control the whole life of the plant. 



It is obvious, however, that we can only realise that which potentially 

 exists in the internal structure of the species. This structure, with all its 

 potentialities, forms the basis of our experiments and can never be explained. 

 But this idea of further potentialities- is what guides us in our quest of 

 new discoveries. The manner of development of a plant under ordinary 

 conditions is only one of its many possibilities. Nature must be much 

 richer tlian appears under the influence of ordinary environment. We seize 

 wpon this idea to create new kinds of environment, and thereby to produce 

 new forms not otherwise occurring. We shall recognise the methods and aims 

 -of the physiology of development better by making ourselves acquainted with 

 .the facts than by long discussions. 



I will first examine a single specimen of the lower plants — the fungus 

 Saprolegnia — before we pass on to tlic higher ones. A white mould, wliich 

 belongs to the fungoid genus Saprolegnia, and to the species mixta, appears 

 very frequently upon dead insects in swau)ps. The fungus consists of tube- 

 like branched filaiiusnts, known as the mycelium, which enter tlie body of 

 the insect with their basal parts, and then spread out in tlie water. For the 

 first few days, vegetative growth rules exclusively, whilst the filaments grow 

 longer and branch. The second stage, characterised by tlie formation of 

 asexual reproducing cells, follows the first. The ends of the filanientH swell up 

 a little and are separated by a partition, producing, by division, nuiucnous very 

 minute cells, known as zoospores. These make their exit through a hole and 

 swim free in the wat(!r until they find a new substratum, in the shape 

 of another dead insect, ujion whicli they can settle. The formation of 

 zoospores takes place again and agiiin on tiie myc'olium for some days. 



Finally, a third stage of developmiiut follows, namely, the formation of 

 sexual organs, male ami female. Sph(!rical cells are formed on the lilaments, 



