SECT. 2] AND WEIGHT 469 



unilaterally nephrectomised animals causes a much increased com- 

 pensatory hypertrophy of the remaining kidney. 



Carnot has also studied the effect of chick and rabbit embryo 

 extracts on the growth of tadpoles, using glycerol extracts or the 

 powdered dry substance. There was no influence on metamorphosis, 

 but a great increase of growth, the experimental tadpoles being 

 two or three times as big as the controls. This recalls the older 

 observations of Springer who maintained that the eggs of Arbacia 

 punctulata and Asterias forbesii contained a substance inhibitory to 

 the growth of other eggs of the same species. The retardation 

 was noted in the early rather than the late embryos, and there 

 was a marked tendency for the eggs to stop development when they 

 arrived at the blastula stage. Subsequent work by Peebles indicates 

 that a growth-promoting factor (perhaps a proteose) may also 

 be found in echinoderm embryos. These observations are not very 

 convincing and perhaps the subject might be re-examined with 

 advantage. 



Experiments which may, when extended further, throw some light 

 on these questions have been made by Skubiszevski, who transplanted 

 chick embryo tissues into adult hens. The complete failures were 

 more or less numerous at all stages of development, and so probably 

 represented errors of technique, but the conspicuous successes gave 

 a curve which was markedly peaked at the 3rd day of development. 

 The grafts grew well without antagonism for as much as 72 days 

 as mesodermal connective tissue, but in three cases (out of 1 78) they 

 resembled a sarcoma at the end of that time. This 3rd day peak 

 may be an important clue. Precisely similar work on grafts was done 

 by Uhlenluth, who transplanted the eyes and skin of larval sala- 

 manders into individuals of various ages. These transplanted organs 

 attained maturity not at the time when their original possessor took 

 on adult characteristics, but at the time when their new possessor 

 metamorphosed. They fell into step, as it were, with their new en- 

 vironment. The hormones and growth-promoting substances which 

 regulate such changes are still imperfectly understood, but Babak; 

 Gudernatsch; Swingle; Huxley & Hogben and many other workers, 

 have fully unveiled the importance which thyroxin has in amphibian 

 metamorphosis. 



Another way of studying the essential nature of the growth-impulse 

 might be to plot the viability for in vitro tissue culture of individual 



