Chapter XIV 



SUMMARY 



§1 



It is now possible to give a brief summary of the chief points which 

 have emerged from our study of development, during which 

 attention was focussed on differentiation and its origin as the 

 central problem. 



In the first place, animal development is truly epigenetic, in that 

 it involves a real creation of complex organisation. It is also pre- 

 determined, but only in the sense that an egg cannot give rise to an 

 organism of a species different from its parent. The development 

 of each individual is unique. It is the result of the interaction of a 

 specific hereditary constitution with its environment. Alterations 

 in either of these will produce alterations in the end result. 



Determination is progressive. In the earliest stages, the egg ac- 

 quires a unitary organisation of the gradient-field type in which 

 quantitative differentials of one or more kinds extend across the 

 substance of the egg in one or more directions. The constitution 

 of the egg predetermines it to be able to produce a gradient-field 

 of a particular type ; however, the localisation of the gradients is 

 not predetermined, but is brought about by agencies external to 

 the egg. The respective roles of internal predetermination and 

 external epigenetic determination are clearly seen in regard to the 

 bilateral symmetry of the egg. The amphibian egg is predeter- 

 mined to be able to give rise to a gradient-field system of bilateral 

 type through the establishment of the grey crescent at a particular 

 latitude of one meridian. The particular meridian is not predeter- 

 mined, but is normally decided by the point of sperm-entry ; the 

 precise latitude is determined as a result of the primary axial 

 gradient of the egg, impressed upon it by factors in the ovary. On 

 the other hand, the egg of a radially symmetrical animal like a 

 Hydroid is incapable of developing bilateral symmetry; the pre- 

 determined capacity to react to stimuli localised in one meridian is 

 not given in its constitution. 



