392 THE FURTHER DIFFERENTIATION OF THE 



The same non-specific attraction has been shown in the case of 

 grafts of rudiments of chick embryos on to the chorio-allantois. If 

 the rudiments include those of the mid-brain, muscle-segments, 

 cartilage, and mesonephros, it is found that nerve-fibres grow out 

 from the mid-brain towards them. Normally, the neurons of the 

 mid-brain produce axons which do not emerge from the central 

 nervous system, but form visual association neurons. Under the 

 conditions of the experiment, however, they are attracted towards 

 the various structures which happen to be differentiating in prox- 

 imity to them.i It may also be noted that in these experiments the 

 mid-brain is not enclosed in a connective tissue capsule, so that 

 there is no mechanical obstacle to the outgrowth of axons. ^ 



It would appear that this non-specific attraction is a result of a 

 high degree of physiological activity on the part of the structure 

 exerting the attraction ; and in a general way the growth of a nerve- 

 fibre towards such a structure may be compared with its growth up 

 and down the gradients within the neural tube. It should also be 

 noted that the deflection of nerves to an abnormally situated graft 

 is greater if the graft is a limb than if it is an eye. 



A structure or organ which is already innervated appears to exert 

 no attractive effect on a growing nerve ; it is, as it were, saturated. 

 This fact emerges clearly from experiments on Amblystoma in 

 which the limb-rudiment is removed and a tail-rudiment is grafted 

 on to the side of the body, some distance behind the normal limb 

 position. Contrary to what happens when a limb, an eye, or a nasal 

 pit is grafted, no nerves grow out towards the tail. This is presum- 

 ably because the tail contains its own little piece of neural tube, the 

 nerves from which provide for its own innervation.^ It must be for 

 this reason that in those cases where a limb is transplanted to an 

 abnormal position, the brachial nerve (which is attracted by the 



^ Hoadley, 1925. 



2 See also Detwiler, 1928 a. A similar alteration of morphological process in 

 the absence of a retaining capsule is seen in the lens. When lens-rudiments are 

 grafted into blastulae, they develop as regular spheroids if their limiting membrane 

 remains intact. If, however, it is locally damaged, a large irregular protrusion of 

 fibre-elements occurs (Kriiger, 1930). In a somewhat similar way, the normal 

 absence of capsule round the thyroid of teleost fish permits a pseudo-mialignant 

 growth of the organ if it is induced to hypertrophy, while this is impossible with 

 the encapsulated thyroid of higher forms (Marine and Lenhart, 191 1). 



^ Detwiler, 1928 b. 



