ORGAN DEVELOPMENT IN VERTEBRATES 267 



during evolution, a phenomenon which is of fairly widespread occur- 

 rence. 



The epigenetics of kidney development has been rather extensively 

 studied; recent reviews are those of Fraser (1950) for the vertebrates in 

 general, Cambar (1948) for Amphibia and Waddington (1952^) for 

 birds. 



In the Amphibia, the capacity to develop into pronephros appears at a 

 certain level of a gradient which runs from a high point in the chorda to a 

 low in the lateral plate. This was demonstrated by Yamada (1940), who 

 showed that if lateral mesoderm is taken from a position in the neurula 

 some distance away from the embryonic axis and cultivated in isolation, 

 it will develop only into tissues normally formed from such lateral regions, 

 such as blood, while if chorda is added to the isolate, pronephric tubules 

 appear (see Fig. 10.10, p. 191). It is to be presumed that the first step in 

 the development of the more posterior intermediate mesoderm (which 

 will form mesonephros) is taken in the same way; and it seems not 

 unlikely that a similar process occurs in the bird embryo, though this has 

 not been defmitely proved (cf. Waddington 1952^). 



There is no evidence that the amphibian pronephros requires any 

 further stimulus, after its position on the medio-lateral gradient is fixed, 

 before being able to complete its development. For the more posterior 

 kidneys (mesonephros and metanephros), however, something further is 

 necessary, namely an inductive influence which is normally exerted by the 

 pronephric duct. As we have seen, this duct grows backwards from the 

 region of the pronephros. If its backward extension is prevented (e.g. by a 

 transverse cut which fails to heal completely) no sign of the duct appears 

 in the posterior region, and only minor traces of the mesonephros develop. 

 The dependence of the amphibian mesonephros on an inductive stimulus 

 from the pronephric duct was first suggested by Miura (1930) and later 

 work by many authors has fully confirmed it (O'Connor 1939, Cambar 

 1948). In birds, Boyden (1927), Griinwald (1937) and Waddington (1938) 

 have found a similar situation (Fig. 12.8). 



In birds the pronephros never functions as an excretory organ, and 

 Waddington suggested that it had been retained in the ontogeny of the 

 animal simply because it is an essential step in the formation of the prone- 

 phric duct which is itself necessary as the inducer of the mesonephros. 

 Cambar (1948) has criticised tliis suggestion on the grounds that in the 

 Amphibia the pronephric duct arises from a mass of tissue wliich is dis- 

 tinct from, although it lies immediately in contact v^th, that which gives 

 rise to the pronephros itself; moreover he states that the growth of the 

 duct is independent of the continued presence of the pronephros, whence 



