54 EARLY AMPHIBIAN DEVELOPMENT 



by the two types of experiment we have mentioned ; the grafting 

 of tissues into abnormal positions relative to an intact organiser, 

 and the grafting of an organiser in abnormal positions relative to 

 an otherwise intact host embryo. 



However, after a certain critical time during gastrulation, the 

 various main regions develop, in respect of the type of tissue they 

 produce, by self- differentiation. A piece of tissue grafted into an 

 abnormal situation no longer has its fate determined by its position 

 in relation to other tissues ; the factors controlling its development 

 are now situated within itself. 



Of course, all differentiation is in certain respects dependent, in 

 others independent. When grafts are made from one species to 

 another before gastrulation, the grafted piece shows dependent 

 differentiation as regards the organs and tissues which it forms, but 

 self-differentiation as regards various fundamental characters such 

 as cell-size and pigmentation (see p. 142). Conversely, in certain 

 respects the fate of a piece of tissue in the self-differentiating phase 

 is dependent on external conditions, for, as we shall see (p. 249), 

 the development of its shape is dependent on mechanical factors in 

 its new situation, whereas the type of tissue which it produces is 

 not. 



In experimental embryology, the terms are generally used in 

 respect of dependence of type of tissue produced upon the activities 

 of other parts of the embryo. Dependence upon external agencies 

 is not usually discussed in this connexion (although some differ- 

 entiations such as that of polarity are dependent upon them), but 

 these are assumed to remain more or less constant, within the range 

 permitting of normal development; and form-differences due to 

 purely mechanical distortion are also usually omitted from con- 

 sideration. Within these limits, the terms will be found very useful. 

 Other examples of self-differentiation are to be found in the 

 development of the organiser region, of the eye-cup and of many 

 other organs mentioned in Chap, vii, and of particular types of 

 tumours and cancers irrespective of their site. Other examples of 

 dependent differentiation which will be met with are the depen- 

 dence of the lens and conjunctiva upon the eye-cup (pp. 178, 183), 

 of the ear-capsule upon the ear- vesicle (p. 175), the dependence 

 of amphibian metamorphosis upon a certain concentration of 



