430 THE PREFUNCTIONAL AS CONTRASTED WITH 



muscles of the tail during its resorption, will produce the same 

 effect, but more slowly. Thus presumably some substance pro- 

 duced during autolysis is the agent responsible (figs. 208, 209). 



§5 

 The next subject to consider is the trophic effects of the nervous 

 system. In view of the fact that innervation (by fibres of the auto- 

 nomic nervous system) is a prerequisite condition for regeneration 

 of limbs to take place in adult newts, it is most interesting and 

 curious to find that the nervous system is not essential for the 

 embryonic development of the amphibian limb. It is difficult to 

 obtain embryos in which the limbs are not supplied by some, even 

 abnormal, nerves, for, as already explained (Chap, xi, p. 389), the 

 limb exerts an attraction on the 

 growing axon. But limb-rudiments 

 have been seen to develop when 

 free of any nerve-fibres. This con- 

 dition can be realised by grafting 

 the limb-rudiment of a frog into 

 a lymph-space of another larva, 

 or by extirpating the neural tube 

 opposite the limb region on one 

 or both sides in the neurula stage. 

 The limbs are normally differen- 

 tiated as regards all their con- 

 stituent tissues and parts : cartilage, 

 muscles, skin, blood-vessels, and 

 the joints between the skeletal The trophic effect of the nervous 



11 , „ system on the development of the 



segments, all these are normally limb. Ventral view of a larva (shortly 

 differentiated in the absence of before metamorphosis) of i^awa/w^c^ 



,• _^ ^^.: 1 „4. .1 _ i:»^u „^ ^ from which at the neurula stage the 



mnervation, but the limb as a .^diment of the lumbo-sacral region 

 whole is too small. ^ In other words, of the spinal cord was extirpated on 



the nerves have a trophic but not a ^^^, "S^t side. Note normal form of 



. . rr 11 right leg but subnormal size and 



morphogenetlC ettect on the de- development. (From Hamburger, 



velopment of the limb (fig. 210). In ^rch. Enuumech. cxiv, 1928.) 

 this respect the effect of the nerve is similar to that of thyroid hor- 

 mone on limb-growth in larval Anura^ (see also Chap, x, p. 363). 



* Lebedinsky, 1924; Hamburger, 1929. ^ Champy, 1922. 



