140 PRINCIPLES OF EMBRYOLOGY 



prothoracic hormone can be analysed to some extent in biochemical 

 terms ; it causes profound changes in the cytochrome enzymes which are 

 concerned with respiration in the pupal tissues. This brings about a 

 complete change in the metabolic system of the insect, and it is clearly as a 

 result of this change that the development of the various organs is once 

 more able to proceed. 



It is perhaps worth emphasising the obvious fact that the metamor- 

 phosis and diapause hormones do not determine what type of development 

 any particular tissue will undergo, since they affect equally all the different 

 rudiments. They act as what have been called 'realisers', which make it 

 possible for potentialities to become actual, but they are not 'determiners', 

 which could change the characters of the reacting tissues. The change they 

 bring about is, at least in the Hemimetabola, to be compared with a 

 modulation (p. 14) rather than a determination. It can be to some 

 extent reversed, since if an adult bug is provided with large amounts of 

 juvenile hormone, it may moult again and the larval characters reappear 

 (Wigglesworth 1948(7, b). 



4. The determination ofimaginal characters. 



In insects with complete metamorphosis, such as the Diptera, the 

 future imaginal tissue is present during larval life in the form of separate 

 pockets of cells, the so-called imaginal buds. These originate from the 

 hypodermis of the embryo, which is part of the ectoderm. In the early 

 stages of larval life, there is some variation in the readiness with which 

 the different buds react to a given concentration of metamorphosis 

 hormone, and it appears that they undergo, at shghtly different rates for 

 different buds, a process of maturation by which they acquire an in- 

 creasing competence to respond (Bodenstein 1943, 1950). There has 

 been considerable debate as to when these buds become determined in 

 their developmental fate. By irradiating Drosophila embryos with ultra- 

 violet, Geigy (193 1) was able to produce purely imaginal defects in ani- 

 mals whose larvae had developed perfectly normally. This occurred only 

 when the irradiation was given about seven hours or more after laying, 

 at a time when the embryo is well on the way to formation. It therefore 

 appears that, long after the period of embryonic determination (which is 

 extremely precocious in Diptera) there is a second period when the 

 imaginal buds are determined. Liischer (1944) has recently produced 

 somewhat similar evidence concerning the Lepidoptera. Evidence tending 

 in die same direction has been brought forward by Gloor (1947) who 

 found that by ether treatment of the young Drosophila embryo he could 

 cause the metathoracic imaginal bud to develop in the way characteristic 



