2l6 PRINCIPLES OF EMBRYOLOGY 



that evocation is dependent on specific rather than imspecific stimuh. 

 But however much we might discover about the chemical differences 

 within the organiser, this would not provide any explanation of the way in 

 which the reacting tissue comes to assume the specific and definite shape 

 of a particular neural organ such as the forebrain, hindbrain or spinal cord, 

 or of how the evocating substances come to be arranged in an orderly 

 pattern within the mesoderm. This is the problem of morphogenesis and 

 individuation. It will be discussed more fully later (p. 455). Here we shall 

 deal only with the simpler chemical problem of the nature of the evocators 

 involved. 



There is considerable evidence (see Holtfreter 195 1), that competent 

 ectoderm of the gastrula has no particular regional properties and that no 

 part of it shows any special tendency to develop either towards the head 

 or the tail. Although Nieuwkoop (1947) fairly recently queried this, he 

 seems to have had little good reason for doing so, other than the needs of a 

 theoretical scheme he was putting forward. If therefore a process of 

 induction shows a regionally specific character, this is to be attributed to 

 the properties of the inducer rather than of the reacting material. 



Early attempts (Lopaschov 1935^7, b) to demonstrate the existence of 

 chemically different evocators in different regions of the mesoderm were 

 rather unsuccessful. However, shortly after this Chuang (1938, 1939, 

 1940) and Toivonen (1940) found characteristic regional differences 

 in the inductions produced by implants of killed tissues from various 

 organs of adult mammals (see also Hama 1944). The subject has since been 

 rather thoroughly investigated by Toivonen (summary 1949, 1950). 

 There is no close anatomical correspondence between the organ from 

 which the implanted tissue is taken and the type of induction it produces. 

 For instance, Chuang found that mouse kidney tended to induce parts of 

 the brain, and Triton liver induced more posterior structures; while 

 according to Toivonen, a guinea-pig kidney is a posterior inductor and 

 guinea-pig liver an inductor of anterior parts. The distribution of the 

 evocator substances in the adult body therefore seems to be rather hap- 

 hazard. 



Toivonen has made some progress towards finding out the chemical 

 properties of the substances concerned. He concludes that there are two 

 main evocators. The property of one is to induce anterior regions of the 

 brain (the so-called archencephalon). This substance is relatively resistant 

 to boiling, is soluble in organic solvents and easily dialysable. The second 

 substance induces parts of the spinal column or the hindbrain (the deuter- 

 encephalon). It is very thermolabile, being destroyed by heating to 

 90° C. and is insoluble in petrol ether. The adult tissues as used in im- 



