314 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



the organism of the caterpillar and the simultaneous differentiation 

 and realization of the latent organism of the butterfly which super- 

 sedes it. 



In the development of animals, the stage of visible differentiation 

 of the parts is preceded by a stage of "determination," at which, 

 although all parts outwardly look alike, each is in fact already com- 

 mitted or determined to form some particular component of the 

 organism that is to be produced. Development is then said to have 

 reached the mosaic stage. Determination of the main parts of the 

 insect body occurs very early in the development of the egg. In some 

 insects, the fruit fly Droso'phila (4)^ for example, the mosaic state is 

 already attained in the germ plasma on the surface of the ^gg at the 

 time of laying; that is, before the single nucleus of the egg has even 

 started to divide. In other insects this may not happen until after 

 the germ band has formed. If, when the mosaic state is reached, 

 a part of the determined area is destroyed by burning, or by irradiation 

 with ultraviolet light, the corresponding part of the insect will be 

 lacking when visible differentiation and development are complete. 



The interesting fact about these insects is that, even at this early 

 stage of development, the larval organism and the adult organism 

 are distinct. Elimination of restricted areas of the newly laid egg 

 of Drosophila results in corresponding deficiencies in the resulting 

 larva — but the adult fly, when it appears, is perfectly normal. At 

 this stage the egg is a mosaic in respect of larval characters, but is 

 still undetermined in respect of the adult characters. But within 7 

 hours after laying, imaginal determination has taken place; the egg 

 is now a mosaic in respect of adult characters also, and injuries to 

 restricted areas at this time become apparent in the adult fly. Indeed, 

 if they affect organs, such as legs or wings, that are not present in the 

 larva, the results of these injuries are not visible at all until after 

 metamorphosis. The same thing is seen in the clothes moth Tineola 

 (5) ; by irradiation at the appropriate moment it is sometimes possible 

 to obtain a clothes moth with normal limbs developing from a larva in 

 which one or more legs were completely absent. 



Thus, metamorphosis consists in the realization of all those adult or 

 imaginal characters that remain latent throughout larval life; the 

 physiological study of metamorphosis consists in the analysis of the 

 factors by which the manifestation of these imaginal characters is 

 controlled. 



Many diverse hypotheses have been put forward in the past; but 

 in recent years evidence has accumulated that control is exercised 

 by means of hormones. For the purpose of experiment it proved 

 convenient, in the first instance, to use a hemimetabolic insect; that 



* Numbers iu parentlicscs iudicate references at end of article. 



