A PRELIMINARY EXPERIMENTAL ANALYSIS 57 



the organiser can become neural folds by self-differentiation. There 

 seem, therefore, to be two methods by means of which neural folds 

 can arise; such a phenomenon is referred to as ''double assurance''. 

 Further discussion of this question is given in Chap, vi 

 (pp. 139, 187). 



As soon as some organs have reached the stage of full self- 

 differentiation, they become able to induce other organs to arise by 

 dependent differentiation. In many forms, for instance, the eye- 

 cup induces the formation of a lens from the overlying epidermis 

 (see p. 183), in a manner analogous to that by means of which the 

 organiser induces the formation of neural folds (fig. 21). How 

 general such secondary induction may prove to be in development 

 is not as yet known. 



However, we do know that in many cases what is first determined 

 is a large region or field, and that later this region becomes split up 

 into a further mosaic of independently determined subregions. For 

 instance, as set forth more in detail in Chap, vii, the limb area is early 

 determined as a region in the flank (fig. 22) : only later are the 

 various subregions, such as hand, forearm, upper arm, determined 

 within the main region. 



During the period of self-differentiation, the embryo is thus a 

 patchwork or mosaic of developing regions, the differentiation or 

 localisation of all of them being originally dependent on something 

 else, ultimately on the axis of polarity and plane of bilateral sym- 

 metry. The differentiation, however, is progressive, the mosaic 

 coming to consist of more and smaller pieces, each of which eventu- 

 ally undergoes independent differentiation. 



At this stage, almost the only integrating influences acting upon 

 the embryo appear to be the simple ones of mechanical construction. 

 Biological integration is almost absent : neither neural nor humoral 

 correlation is yet possible, and little trace has been detected of 

 influences analogous to that of the organiser or the optic-cup, or 

 of chemical influence by contact. The chief exception appears to be 

 that the polarity of the egg may persist to cause the polarisation of 

 some on all of the separate organ-rudiments (see Chaps, vii and x). 

 The embryo at this stage is like a multiple tissue-culture, the parts 



