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INSECT METAMORPHOSIS: 



AN APPROACH TO 

 THE STUDY OF GROWTH* 



Carroll M, Williams 



HARVARD UNIVERSITY 



The complex happenings during embryonic and post-embryonic de- 

 velopment have one distant but overriding objective— the production 

 of adult, sexually competent individuals. With the exception of the 

 higher amphibia, most vertebrate animals approach this goal in what 

 may be termed a straight line. So, in the case of the human species, 

 embryonic development gives rise to an infant whose resemblance to 

 the adult is self-evident. The infant is a logical starting point for the 

 construction of the adult. 



But among the vast majority of invertebrate animals, especially 

 those that live in the sea, the goals of embryonic development are far 

 different from the formation of diminutive adults. Thus, in most species 

 of marine worms, molluscs, and Crustacea, and in all echinoderms and 

 protochordates, embryonic development gives rise to tiny larval forms 

 having little in common with the adults that produced them. As 

 pointed out by Snodgrass ( 1954 ) , metamorphosis has its roots in this 

 deviation in the course of embryonic development away from the 

 pathway leading to the adult. 



Characteristically, in the marine invertebrates and protochordates 

 the larval stage or stages are transient. The larvae are spread about by 

 the current of the sea; their biological role is to be broadcast. Then, 

 still as microscopic objects, they discard their larval organization, 

 metamorphose to parental form, and proceed to grow to adult size 



* The studies described here were supported in part by a grant from the Di- 

 vision of General Medical Sciences of the National Institutes of Health. 



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