440 SUMMARY 



cyclopia in fish, modification of regeneration in Planarians). Here 

 again, there is a predetermined capacity to produce a certain type 

 of structure in certain conditions ; but the precise locahsation of the 

 structures produced depends upon the form of the gradients in the 

 field-system. 



Once the mosaic stage has set in, further diff"erentiation may be 

 brought about by the influence of one point on its neighbours. The 

 classical example of this is the induction of a lens from epidermis 

 by the optic cup. 



During the period when the organisation of the developing 

 animal consists of a single field-system, far-reaching regulation is 

 possible; after irreversible chemo-differentiation has occurred, it 

 is not. The precise time at which irreversible chemo-differentiation 

 sets in varies markedly in different groups. In Amphibia it occurs 

 during gastrulation ; in Ascidians at fertilisation. 



After the establishment of a mosaic of partial fields, it does not 

 follow that all the cells of any given partial field necessarily give rise 

 to the organ characteristic of the field. Thus, more cells are capable 

 of giving rise to the amphibian fore-limb than do in fact give rise 

 to it in normal development. Further, the boundaries of the partial 

 fields overlap : a given group of cells in the limb-rudiment of the 

 chick may contribute to the formation of either a thigh or a shank, 

 according as to whether it is allowed to remain attached to or is 

 isolated from one partial field or the other. Gradients may exist 

 in such fields : the capacity of cells within the fore-limb field to give 

 rise to a limb decreases with their distance from a subcentral 

 portion of the field: the same is true for many other organ-fields. 



§3 



Up to a certain time, regulation is still possible within each of the 

 partial fields ; but as development proceeds, each of these becomes 

 split up into progressively smaller fields, each with its own deter- 

 mined fate : for instance, the fields for leg, shank, and foot, within 

 the originally single hind-limb field. 



Each area in the mosaic passes from the state of invisible chemo- 

 differentiation by the process of histo- differentiation to full visible 

 differentiation, and so reaches the functional stage. After the 

 organism as a whole has reached the functional stage, many new 



