350 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



We may here mention some other experimental resuhs which 

 may be interpreted on similar lines. It has been seen (p. 156) 

 that a constriction of the blastula of the newt in the plane of sym- 

 metry will lead to the formation of two miniature but complete 

 embryos if the constriction is complete, or of a double monster in 

 which there are two perfectly formed heads joined on to a single 

 posterior region of the body, if the constriction is incomplete. 

 Sometimes, however, the plane of the constriction is not exactly 

 coincident with the plane of bilateral symmetry, and one half 

 comes to contain more of the region of the animal pole (i.e. the top 

 of the gradient) than the other. In such cases, while one of the 

 heads of such a monster is normal, the other is cyclopic ^ (fig. 169). 



The explanation is based on the same considerations as those 

 already used above. Since by the constriction, one half has been 

 deprived of the region of the extreme animal pole, that half has a 

 gradient of which the top is not relatively high enough to form a 

 perfect head, complete with extreme apical structures ; the other half, 

 with the complete gradient, is capable of doing this (see fig. 170). 



Certain lines of evidence indicate that it is the high point of 

 the organiser gradient which is affected by lithium, not the high 

 point of the eggs' primary gradient. ^ 



§8 



From what has already been said in regard to power of regulation, 

 either in isolated blastomeres (p. 102) or in particular organ-fields, 

 it should now be clear that regulation in early ontogeny can only 

 occur while the system in question is in the form of a gradient- 

 field : it cannot occur when the system is split up into a mosaic of 

 independent chemo- differentiated regions. A system, be it egg, 

 blastomere, or field, can only make good the loss of material in so 

 far as that which was lost only formed part of a field, and was not a 

 definitely localised determination forming part of an established 

 mosaic. In regeneration, the new dominant region may override and 

 remodel what remains of the original organisation. 



From this point of view, power of regulation ceases to be a 

 mysterious force striving for a return to the normal : systems that 

 can regulate are merely in the same case as the egg, viz. gradient- 



^ Spemann, 1904. ^ F. E. Lehmann, 1933, Rev. Suisse Zool. XL, 251. 



