56 EARLY AMPHIBIAN DEVELOPMENT 



thyroid hormone (p. 427), the dependence of the fine structure of 

 bone upon the functional stresses to which it is exposed (p. 434). 

 The lateral line in tadpoles is independent as regards its histo- 

 logical differentiation and increase in size, but dependent in regard 

 to the position it comes to occupy (p. 355). 



In first origin, each process of differentiation is dependent. As 

 we have seen, the differentiation of an axis of polarity is dependent 

 on factors in the ovary ; the differentiation of bilateral symmetry is 

 normally dependent on the point of entry of the sperm ; and the 

 localisation and determination of the organiser itself is dependent 

 on both the axis of polarity and the plane of bilateral symmetry, for 



Fig. 22 



Mosaic stage: localised determination of limb-potencies. Left: Amblystojna 

 embryo immediately after removal of the right fore-limb field. The pronephros is 

 seen in the wound-area. Right: a larva on which a similar operation has been 

 performed, but on the left fore-limb field, 3 months later. There is no trace of 

 a left fore-limb. (From Harrison, Proc. Nat. Acad. Sci. i, 191 5.) 



it arises in the latter and at a particular level (or parallel of latitude) 

 with regard to the former. The differentiation of all other regions 

 is dependent on some presumably chemical action of the organiser 

 and on their position relative to the axis of polarity and the plane 

 of bilateral symmetry, though in a manner which is still obscure 

 (see Chap. ix). 



The case of the neural folds raises a problem of particular in- 

 terest, for, as has been mentioned, not only can neural folds arise 

 by chemo-differentiation in situ even if the organiser is removed or 

 prevented from invaginating, but also the organiser is capable of 

 inducing the formation of neural folds wherever it is grafted. In 

 normal development, that tissue which the organiser normally in- 

 duced to become neural folds is also that which in the absence of 



