188 



LIFE: ITS NATURE AND ORIGIN 



Then it was found that a dead tissue, a chemical fraction or 

 a substance of known constitution could serve as an inductor; 

 and J. Holtfreter 8 found that certain tissues which when alive did 

 not induce, acquired this power after being boiled. The term 

 inductor is therefore much broader than organizer, which is 

 limited to living tissue. The chemical substance emitted by an 

 organizer and responsible for all or part of its morphogenetic 

 stimulus was termed by Needham an evocator; its action is evo- 

 cation. 



Gastrula invagination 



Organisation centre 

 /v 



SZ 



r. 



Head-organiser" 



' Tail-" or " Trunk -organiser ' 



Head endodenm 



Head mesoderm 



r 



Notochord 



Somites 



Fore-brain Mid-brain Hind-brain 



Gills Mouth Teeth 

 opening 



r 



Side-plate 

 mesoderm 



"N 



Gut lumen / Neural tube 



Eye-cup Ear-vesicle Balancer Spinal ganglia 



and mesenchyme 



Frontal Nasal 

 glands grooves 



Lens Ear-capsule Placodes Dorsal fin 



Cornea Tympanic 

 membrane 



Ectodermal 

 mesenchyme 



Skin Limb buds 



Figure 30. The succession of inductors of second and lower grades. (From Need- 

 ham's "Biochemistry and Morphogenesis," Cambridge University Press, Cambridge, 

 England, 1942.) 



But it has been found that stimuli of this kind will be ineffective 

 unless the tissues on which they act are properly receptive. This 

 state of reactivity is known as competence? Competence and organizer 

 activity are not retained indefinitely, but may arise and fade away at 

 different times; however the latter appears to begin sooner and to 

 persist longer. Needham states 9a that no tissue is known which 

 possesses competence but no evocator. 



While the terms "competence," "organizer activity," "morphological 

 stimulus" express the facts, they fail to give any inkling of the opera- 

 tive mechanism, which we believe is more nearly approached by the 

 several catalyst mechanisms mentioned below. Thus a tissue may be 

 "competent" to supply a carrier for the prosthetic group of an 

 organizer, or vice versa; and these two units may form a specific and 

 active catalyst or enzyme, providing that all conditions (e.g., pH, 

 absence of inhibitors) are favorable. If competence involves, for 

 example, the presence of a labile but powerfully adsorbent protein 



