306 AUGUST WEISMANN [apf.il 



when amputated, but the reverse is true in regard to the gills, which, in 

 these forms, are not infrequently bitten off. The latter are physio- 

 logically important organs, but this can no longer be said of the former. 

 On the other hand, the strong and biologically indispensable limbs of 

 the less elongated newt (Triton) have a high regenerative capacity. 



The fact that internal organs not naturally exposed to mutilation 

 are not regenerated in animals which show great regenerative power 

 as regards external structures is in harmony with this view. Halved 

 lungs in Triton do not grow again, but simply close up, and I can 

 now add to this the fact that the oviduct and the vas deferens are 

 neither renewed nor even elongated by fresh growth if a portion of 

 them be excised. The experience of pathologists is also in agreement 

 with this, for they report that they have never seen the regeneration 

 of an experimentally excised lobe of liver or kidney in any animal, but 

 only an increased function in the remainder of the organ — in short, 

 there is no morphological regeneration but only a physiological com- 

 pensation. On the authority of my renowned colleague in pathological 

 anatomy, Ernst Ziegler, 1 I may add that, in man, the power of re- 

 generation is possessed only by such tissues and portions of tissues as 

 are subject, in the normal course of life, to continual or periodical 

 wearing out. Such are, the mucous membrane of the intestinal tract, 

 the epidermis of the skin, the whole mucous membrane of the uterus 

 with its glands, etc. To these must be added certain tissues which are 

 very frequently but not regularly or periodically liable to injury, and 

 are at the same time of great biological importance. To this category 

 the cutis belongs. In it, however, as is well known, complete histo- 

 logical renewal does not take place, but only a partial reparation 

 by means of cicatrisation. The scar-tissue — which is also formed 

 from connective tissue, but contains no skin-glands, and has a different 

 arrangement of the connective-tissue strands — is well adapted to the 

 closing of small skin wounds, and quite sufficient to re-establish in 

 approximate integrity the important protective adjustments of the cutis. 

 Regeneration never occurs, on the other hand, either in nerve-cells of 

 any kind whatever, or in the supporting (neuroglia) cells of the brain, 

 notwithstanding their great functional importance. All these facts 

 are in harmony with the theory suggested, that the power of regenera- 

 tion possessed by an animal or a part of an animal is regulated by 

 adaptation to the frequency of loss, and to the extent of the damage 

 caused by the loss. Hitherto, however, it seemed impossible to re- 

 concile with this proposition the case communicated by Kennel, 2 to 

 which I have already elsewhere called attention, 3 of the " stork the 



1 E. Ziegler, " Lehrbuch der allgemeinen Pathologie und der pathologisclien Anatomie," 

 Jena 1895. But the above statements are based on an unpublished lecture delivered by 

 Ziegler in 1898 before the " Naturforschende Gesellschaft " in Freiburg-i-Br. 



2 Kennel, " Ueber Theilung und Knospung der Thiere," p. 18. Dorpat, 1888. 



3 "Keimplasma." (Eng. Ed. p. 125.) 



