Section XII 

 METAMORPHOSIS 



WILLIAM ETKIN 



PosTEMBRYONic development in higher ani- 

 mals generally involves extensive changes 

 in form. However, the term "metamorpho- 

 sis" is not used for such changes unless their 

 occurrence is concentrated in a relatively- 

 short period and involves a striking altera- 

 tion in form. Even then, if the changes are 

 related to sexual development we speak of 

 sexual maturation or puberty rather than of 

 metamorphosis. From a descriptive stand- 

 point, therefore, metamorphosis may be de- 

 fined as a definitely delimited period in 

 postembryonic development during which 

 marked developmental changes in non-re- 

 productive structures occur. Of course, inter- 

 grades between metamorphosis and gradual 

 development occur. Metamorphosis is wide- 

 spread in the animal kingdom. However, the 

 treatment in this section will be limited to 

 the amphibians and insects, two groups in 

 which experimental analysis has been car- 

 ried furthest. 



The adaptive value of metamorphosis lies 

 in the specialization of the larval form to 

 one mode of life and of the adult to another 

 (Figs. 217 and 220). Metamorphosis effects 

 the transition from one type of specialized 

 body form to the other. For the experimenter 

 it is important to recognize that the evolu- 

 tionary process producing divergent speciali- 

 zation between larva and adult has occurred 

 repeatedly and independently in different 

 animal groups. There is therefore no a priori 

 reason for expecting homology between the 

 mechanisms in the different groups. On the 

 other hand, the nvimerous independent mech- 

 anisms controlling metamorphosis which 

 have been developed in the various animal 

 groups may be expected to display analogous 

 characteristics. In this section we shall exam- 

 ine the different mechanisms regulating 

 metamorphosis with the view of discovering 

 the similarities between them in regard to 

 the nature of the controlling stimulus, the 

 role of specificity of tissue response, the role 



of induction, and the role of quantitative 

 variations and other physiological factors. 



ENDOCRINE CONTROL OF AMPHIBIAN 

 METAMORPHOSIS 



The metamorphosis of the amphibian has 

 been the subject of a large number of in- 

 vestigations. Many of these, however, do 

 not fall within the scope of this discussion 

 since in them metamorphosis has been used 

 merely as an indicator of hormonal activity 

 in an investigation whose primary aim has 

 been the study of the chemistry and pharma- 

 cology of various glands. We shall confine 

 our attention to researches which throw some 

 light on the mechanism by which metamor- 

 phosis is governed in the normal animal. A 

 number of reviews of the literature from 

 various aspects are available (Allen, '29a; 

 Schulze, '30; Gudernatsch, '33; Allen, '38; 

 Needham, '42). 



ROLE OF THE THYROID 



The first evidence indicating that amphib- 

 ian metamorphosis is dependent upon thyroid 

 hormone came from the feeding experiments 

 of Gudernatsch ('12). By feeding dried prep- 

 arations of various vertebrate glands to tad- 

 poles he found that only the thyroid, of the 

 many substances used, precipitates within a 

 few days a precocious metamorphosis. These 

 observations have been confirmed and ex- 

 tended by numerous workers who have used 

 whole thyroid of various sources on lu-odele 

 and anuran amphibians (Allen, '29a, '38). 



Extracts of the thyroid gland are also effec- 

 tive and may be administered by injection or 

 merely by keeping the animals in solutions 

 of the material. Although considerable in- 

 formation is available on the chemistry and 

 pharmacology of thyroid preparations, it is 

 sufficient here to note that a very active pro- 

 tein fraction, thyroglobulin, can be extracted 



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