392 CELL-DIVISION AND DEVELOPMENT 



echinoderm-eggs may be stimulated to division by treatment with 

 weak solution of strychnine, sodium-chloride, and other reagents, the 

 result being here more striking than in the case of Chcetoptems, since 

 the entire mitotic system is formed anew under the chemical stimulus. 

 The climax of these experiments is reached in Loeb's artificial pro- 

 duction of parthenogenesis in sea-urchin eggs by treatment with dilute 

 magnesium chloride. Beside these interesting results may be placed 

 the remarkable facts of gall-formation in plants, which seem to leave 

 no doubt that extremely complex and characteristic abnormal growths 

 may result from specific chemical stimuli, and many pathologists 

 have held that tumours and other pathological growths in the animal 

 body may be incited through disturbances of circulation or other 

 causes resulting in abnormal local chemical conditions. 1 



But while we have gained some light on the immediate causes of 

 division, we have still to inquire how those, causes are set in opera- 

 tion and are coordinated toward a typical end ; and we are thus 

 brought again to the general problem of growth.- A very interesting 

 suggestion is the resistance-theory of Thiersch and Boll, according 

 to which each tissue continues to grow up to the limit afforded by 

 the resistance of neighbouring tissues or organs. The removal or 

 lessening of this resistance through injury or disease causes a resump- 

 tion of growth and division, leading either to the regeneration of the 

 lost parts or to the formation of abnormal growths. Thus the 

 removal of a salamander's limb would seem to remove a barrier to 

 the proliferation and growth of the remaining cells. These processes 

 are therefore resumed, and continue until the normal barrier is re- 

 established by the regeneration. To speak of such a "barrier" or 

 " resistance" is, however, to use a highly figurative phrase which is 

 not to be construed in a rude mechanical sense. There is no doubt 

 that hypertrophy, 1 atrophy, or displacement of particular parts often 

 leads to compensatory changes in the neighbouring parts ; but it is 

 equally certain that such changes are not a direct mechanical effect 

 of the disturbance, but a highly complex physiological response to it. 

 How complex the problem is, is shown by the fact that even closely 

 related animals may differ widely in this respect. Thus Fraisse has 

 shown that the salamander may completely regenerate an amputated 

 limb, while the frog only heals the wound without further regenera- 

 tion. 2 Again, in the case of ccelenterates, Loeb and Bickford have 

 shown that the tubularian hydroids are able to regenerate the ten- 

 tacles at both ends of a segment of the stem, while the polyp Cerian- 

 thus can regenerate them only at the distal end of a section (Fig. 194;. 



1 Cf. p. 97. For a good discussion of this subject, see E. Ziegler, '89. 



2 In salamanders regeneration only takes place when the bone is cut across, and does not 

 occur if the limb be exarticulated and removed at the joint. 



