PRINCIPLES OF EMBRYOLOGY 



whereas more posterior parts do not do so unless they are near the head 

 end of the host. This latter point indicates again an mteraction between 

 the host and graft, and in the chick this is sometimes very obvious When 

 a host embryo and an induced one lie closely side by side, they often tit 

 exactly, with each of the organs (head, ears, foregut, heart, etc.) at the 

 same level in each axis, and with the somites exactly Imed up (Fig. io.7j • 

 One can find a complete series between entirely separate axes, lymg 

 some distance apart, through cases where they are closer and show some 

 degree of fitting, to instances where they have so completely united as to 

 form an almost unitary embryo, whose double origm may be difficult to 

 recognise. However, the experiment described above, m which two 

 epiblasts were placed face to face, shows conclusively that mduction is 

 not necessarily dependent on a tendency for a part of the orgamser to 

 expand itself into a complete organ or embryo; in that experiment, both 

 streaks were quite complete as regards ectoderm and mesoderm, lackmg 

 only their endoderm, whereas what they induced was not the missing 

 endoderm, but was the ectodermal neural system (and probably some 

 mesoderm) which they already possessed. 



3. Evocation and individuation 



Facts such as these show that one must take account of two aspects of 

 induction, which will have to be explained by two somewhat separate 

 physiological mechanisms because they can be caused to occur mdepen- 

 dently of one another. The first of these aspects is the mere callmg forth 

 of some sort of an induced differentiation-a process which was origmaUy 

 called 'mduction-as-such' and later 'evocation' (Needham, Waddington 

 and Needham I934).' This is mdependent of any tendency towards the 

 formation of a complete orgamc unit, and is, for instance, weU exemph- 

 fied in the appearance of the secondary embryos in the two-epiblast 



experiment. , 



The second aspect is the formation of an organised structural entity, 

 which may be a whole embryo, or a part of it such as a smgle organ ; tor pro- 

 cesses of this kind the name 'individuation' was suggested (Waddington 

 and Schmidt 1933)- The distinction between these two types of process 

 is quite fundamental for any attempt to formulate theories of develop- 

 ment which penetrate deeper than the special embryological level to the 

 underlying biochemical or genetical fundamentals. It is important to 

 reahse that the characteristic of evocation is not that the response to it is 

 the production of a small or indefinite rudiment (as suggested by Holt- 

 freter 1951) or one which has no definite polarity or structure (cf. Need- 

 ham 1942, p. 126). On the contrary an evocation may sometimes cause 



