140 AN INTRODUCTION TO MODERN GENETICS 



second kind, in which alterations of development are possible in the 

 early stages, are known as "regulation eggs." 



In both regulation and mosaic eggs the most fundamental factor in 

 early development is the arrangement of the different regions into a 

 definite pattern. In addition, in regulation eggs the determined regions 

 are themselves formed by a process of interaction. In vertebrates this 

 interaction can be further analysed. The determination is effected in 

 accordance with the position of the region relative to a controlling 

 region (the organization centre) situated near the blastopore, and the 

 whole pattern of the egg is derived from a simpler pattern which is 

 already present in the organization centre. Patterns of this kind are 

 referred to as "individuation fields." The elements w^hich are arranged 

 in the patterns are the determined regions, or originally, the processes 

 by which the regions become determined. These processes are inter- 

 actions between the indifferent parts of the egg and substances 

 (evocators) given out by the organization centre. The indifferent regions 

 are only capable of reacting with the evocators during a comparatively 

 short period when they are said to be "competent." 



In a vertebrate embryo we have therefore a system of three parts. 

 Competent parts of the egg react with evocators to give regions which 

 are determined to develop in certain ways ; and the organization centre, 

 which gives out the evocators, also possesses a pattern (individuation 

 field) which controls these reactions so that the determined regions are 

 arranged in a pattern to give normal development. A further insight 

 into the nature of the individuation field can be gained from the fact 

 that a part of an organization centre, when isolated, develops not only 

 into that part of the embryo which it would normally form, but into 

 more; in favourable circumstances it may develop into a whole embryo. 

 Similarly, two organization centres, placed together during the time 

 when they are active in inducing development, may amalgamate and 

 induce only a single complete embryo. The pattern of a whole embryo 

 is therefore an equilibrium to which parts of the organization centre 

 tend to return. The organization centre must consist of different 

 regions which arrange themselves in a pattern which is the equilibrium 

 resulting from their mutual interactions. 



An essentially similar scheme may be appUed to other regulation 

 eggs, but in most cases we cannot yet analyse the reactions into com- 

 petent tissues and evocators; we only know that processes of deter- 

 mination go on in definite patterns (e.g. echinoderms, some insects). In 

 mosaic eggs, our knowledge is even less complete. Determined regions 



