SECT. 3] AND ORGANISATION 573 



even come from different genera, families, or sub-classes. Obviously 

 the region of the dorsal lip has in different embryos something very 

 much in common, which can act in a most remarkable way on 

 neighbouring tissues. More extraordinary still, the influence is not 

 a peculiar property of the blastopore lip, for other tissue can be 

 grafted into it, and then can be used as an organiser after having 

 been, as it were, "infected". 



We have already seen that the organiser requires contact for the 

 spread of its influence and will not function across a cut. The spread, 

 Spemann found, also takes a measurable time, for at an early stage 

 the ectoderm near the organiser is determined to turn into noto- 

 chord, while further away it is still indifferent. Ruud & Spemann 

 threw light on the finding of Wilson already referred to when they 

 observed that, if the newt gastrula is divided into halves, only that half 

 which contains the organiser will develop. Brandt has thrown further 

 light on the process of chemodifferentiation and has divided it up 

 into three periods or phases, reversible, critical, and irreversible. 

 Thus different amphibian species which look externally in the same 

 developmental stage may or may not be equally chemodifferentiated. 



The influence that radiates from the dorsal lip is not apparently 

 under the control of any other internal factor; it is therefore called 

 an "organiser of the first grade". But some of the organs which 

 differentiate under its influence show in their turn an organising 

 action on the structures developing near them; thus, the axial struc- 

 tures exert an effect on the development of the thoracic and abdominal 

 viscera [situs inversus viscerum experiments). They are therefore 

 called, as Spemann suggested, "organisers of the second grade". 

 In the same way the eye-cup is usually, though not always, an 

 organiser of the second grade with respect to the lens, as Spemann; 

 Lewis, and many others have shown. 



There is certainly no doubt that some organs at a certain stage 

 contain within themselves all the factors necessary for their complete 

 development as far as the beginning of the third period, while others 

 do not, and seem to depend upon adjacent organs. So far only a 

 few of these relationships have been worked out, but it is very 

 probable that the factor of time is an important one, and, while 

 at one stage an organ or part may be dependent upon some neigh- 

 bouring second-grade organiser, at another stage it may be quite 

 self-differentiating. Examples of these relationships can be found in 



