II. THE PERIOD OF ORGAN DEVELOPMENT 149 



that the originally equivalent cells begin to differ in potencies. 

 Once this has happened, each part of the primordium can 

 produce one part of the organ only, e.g. only a retina, instead 

 of a whole eye. At this stage the parts can no longer replace 

 one another; disturbances and displacements of material within 

 the primordium are no longer regulated. The organ-forming 

 potency can no longer be transposed, but is tied to definite cell 

 groups. Following Paul Weiss, (1926), we may call these 

 happenings ''autonomisation" because they make the parts of 

 the organ autonomous so that from now on they behave as 

 mutually independent units. In the primary organisation-field 

 of the embryo, too, such a process of autonomisation takes 

 place. We have seen that it was this process that led to the 

 formation of the various organ primordia. Viewed in this light, 

 development may be regarded as a stepwise subdivision of the 

 primary field. The first step leads to autonomisation of the 

 organ primordia within the field, breaking it up into a number 

 of more or less independent organ-fields. The second step is the 

 division of the organ-fields into their parts. These parts, in 

 their turn, behave at first as organisation-fields. Experiments, 

 e.g. on the chick embryo, have shown that the field of the leg 

 becomes divided into the sub-fields "upper leg", "lower leg", 

 and "foot". On transplantation of any given part of one of 

 these sub-fields, the graft no longer forms a whole leg, as in 

 the previous stage, but only the appropriate part of the limb 

 (Murray, 1926). It may be assumed that at a still later stage, 

 these sub-fields will again be subdivided by "autonomisation" 

 so that, e.g., the field of the foot will break up into tarsus, 

 mid-foot and toes. At present, however, no information is 

 available on this point. 



The mutual contact inductions of the parts of the germ, and 

 the autonomisation of the parts within the organ-fields, result 

 in a further increase of the spatial multiplicity of the embryo. 

 More and more, the embryo is broken up into a mosaic of cell 

 groups which differ from each other in physico-chemical 

 constitution. This will soon express itself in the external 

 appearance of the cells. Starting from the more or less in- 

 different embryonic cell type, the cells now begin to specialise 



