l8 PRINCIPLES OF EMBRYOLOGY 



field is dependent on its relations with its neighbours, we must imply that 

 those neighbours influence it in a way somewhat similar to that involved 

 in evocation. In fact, one might conclude that ooplasmic segregation and 

 evocation are the processes which occur in those aspects of development 

 which involve sharp and clear-cut differences, such as the formation of 

 different types of tissue, while field phenomena are found when the differ- 

 ences are blurred and intergrading as they are between the various parts 

 of a single harmonious organ. This gives one hope that eventually it will 

 be possible to see all three types of mechanism as mere variants of some 

 more general type; but it is still too soon to attempt to do that, at any 

 rate in an elementary discussion. (For a further discussion of embryonic 

 fields, see the Appendix to this Chapter.) 



It is probable that in every kind of egg, all these three types of process 

 occur, although in some of them one type will predominate, in others 

 another. Moreover, at one and the same time in embryonic develop- 

 ment, processes of different types may be proceeding together. For in- 

 stance, at the time of gastrulation in the Amphibia, the future mesoderm 

 interacts with the ectoderm with which it is being brought into contact, 

 and evocates neural tissue from it; but at the same time a field process is 

 operating, by which not only is the mesoderm moulded into a full set of 

 organs, but the newly evocated neural tissue is brought into the system 

 too, so that a complete and harmonious embryo results. The name 

 'induction' is used for the whole of this complex process, of which evo- 

 cation and field phenomena are separate aspects. 



During the twenties and thirties the ideas sketched above were a suffi- 

 cient guide to lead embryological research into ever new territories ; and 

 there are still many areas of the unknown to which they can unlock the 

 doors. But during the last decade or so it has become increasingly clear 

 that something further is required. The time has come to fmd some point 

 of view which will suggest methods of attacking the problems of the 

 nature of the interactions between ooplasms and nuclei, and between 

 inducing and induced tissues or the different parts of a field. Broadly 

 speaking, two main new approaches are being developed at the present 

 time; one which is biochemical and cell-physiological, another which is 

 genetical. The former is the more direct derivative of previous embryo- 

 logical thought. It seeks to identify and study the biochemical processes 

 which play a crucial role in determination and differentiation, and to 

 discover the nature and functions of intra-cellular structures which are 

 important in this connection. Examples are the study of the physiology of 

 organiser action (p. 193 et seq.), and the investigations on the biochemistry 

 of the gradients in sea-urchin eggs and the role of the mitochondria. 



