752 LIFE: OUTLINES OF GENERAL BIOLOGY 



embryo, e.g. with very rapid cell-division. When this is considered 

 along with the facts of axial gradients, say in Planarian worms, 

 it seems likely that the great intensity of activity has to do with 

 the organising power. 



The nerve-cord and notochord are normally developed in the 

 meridian of the dorsal lip of the blastopore. By excision and 

 re-implantation of the dorsal lip the "axial organs" may be evoked 

 in a region where they would not be normally developed. Or the 

 excision of the "organiser" and its transplantation into another 

 embryo may be followed by the appearance of the usual axial 

 structures, even in irrelevant positions. 



Moreover, transplants from a frog {Bomhinator) or from an 

 Axolotl (Amblystoma) may operate as influential organisers in a 

 newt embryo. There is something unique in the intensely active 

 dorsal lip zone, but it is difficult to explain the uniqueness. 



This difficulty is increased by the fact that if a portion of tissue 

 from some other area, even of a different embryo, be introduced by 

 grafting into this dorsal lip, it can afterwards be excised from it, 

 and be used as an organiser for a third embryo. But here, for the 

 time, the experiments leave us with the question of the nature of 

 this influence. Is it hormonic (or rather pre-hormonic!), or what? 



From experiments such as those which we have illustrated it is 

 beginning to be possible to draw conclusions, embryology in the 

 true sense rising by experimental methods out of embryography. 

 In many cases it is clear that the nucleus with its chromosomes is 

 vastly more important than the surrounding cytoplasm, for an 

 intact nucleus with a minute fragment of the egg-cell may develop 

 into a normal embryo, as Delage's merogony experiments show. 

 In other cases, as in the Tunicate Clavellina, there is evidently an 

 early arrangement of building material in the egg-cell, for micro- 

 scopic vivisection has shown that the removal of a certain segment 

 will result in the absence of a certain structure in the larva. When 

 experimental disturbances have greatly altered the arrangements of 

 the segmentation-cells there may be a striking re-institution of 

 order, so that the outcome is normal. There are indubitable regulative 

 processes in development, as Spemann's work so well shows. One 

 part of an embryo may exert a demonstrable diffusive influence on 

 tissues round about, even when these are not the normal tissue- 

 environment, as is graphically seen when an excised optic vesicle 

 is re-implanted at an irrelevant place. 



ORGANIC REGENERATION 



The term regeneration is usually and correctly applied to the 

 replacement of lost parts of organisms, whether organs or tissues, 



