340 



Embryogenesis: Progressive Differentiation 



potentialities become realized is decided 

 much later in the course of development, 

 apparently at or near the site these cells 

 finally occupy. 



PROGRESSIVE FIXATION OF CHARACTERS 



The entire process of embryonic deter- 

 mination, it will be noted, is a step by step 

 affair. First, a general body zone is mapped 

 out (differentiation center). This zone sub- 

 divides into smaller more specialized zones, 

 with less power of change (germ layers). 

 Progressive specialization continues, bring- 

 ing about a more detailed subdivision of the 

 various embryonic zones into organ fields. 

 Segregation of material, and with it re- 

 striction of potencies, is now so advanced 

 that the different organ fields can no longer 

 replace each other. Yet the capacity for 

 limited regulation might still prevail in 

 certain organ fields even at relatively late 

 stages of development (Liischer, '44). Soon 

 the power of regvilation subsides even in 

 these fields; finally, at the time of visible 

 organ segregation all power of regulation 

 is apparently lost. 



The organism developing in the egg is 

 the larval form of the species. It is the de- 

 termination of its parts that has been dis- 

 cussed so far. When the fully developed 

 larva leaves the egg, it carries in its bodv 

 (holometabolous insects) reservoirs of cell 

 groups from which many structures of the 

 adult animal take their origin. These groups 

 of cells are called imaginal discs; they repre- 

 sent the primitive anlagen of the future 

 imaginal organs, such as legs, wings, eyes, 

 etc. Not all these discs are morphologically 

 distinct at the time the larva hatches from 

 the egg. The time of their appearance varies 

 greatly within different insect groups and 

 even within the various organs of the same 

 individual. In Lepidoptera, for instance, the 

 imaginal wing discs are already visible in 

 the voimgest caterpillar stage, while the 

 leg discs make their appearance only in the 

 last larval instar (Bodenstein, '36). In Dip- 

 tera. both leg and wing discs are visible as 

 small cell aggregations at the time the 

 larva hatches (Auerbach, '36). The regions 

 in the larval body from which these various 

 imaginal discs arise are mapped out long 

 before the discs become actually visible 

 (Bodenstein, '36). Now the evidence sug- 

 gests that the determination of these imaginal 

 regions already occurs in embryonic de- 

 velopment, and that it takes place a short 

 time after the larval structures in the egg 



are determined (Liischer, '44; Geigy, '31). 

 The insect egg, as it has been said, thus 

 becomes the carrier of a double embryo — 

 a larval and an imaginal embyro (Geigy, 

 '31). The imaginal anlagen or their pre- 

 sumptive regions may remain in an un- 

 differentiated embryonic state till very late 

 in postembryonic development and maintain 

 throughout this period a remarkable degree 

 of regulative capacities. The different ele- 

 ments of the imaginal organs become deter- 

 mined at different times. The character of 

 the scales in adult Lepidoptera legs or the 

 type of cuticle sheath formed by the legs 

 at pupation is already determined in the 

 third instar larva while at this time the 

 presumptive materials for femur, tibia, or 

 tarsus can still replace each other. The 

 determination of these latter parts appar- 

 ently takes place shortly before the leg 

 discs become visible at the last larval instar 

 (Bodenstein, '37). The appearance of the 

 imaginal discs per se does not mark the 

 final state of determination, for in Droso- 

 phila, where the leg discs are present in the 

 first instar larva, isolation experiments 

 showed that the discs of young last instar 

 larvae are still capable of regulation (Vogt, 

 '46), reaching their final mosaic state in 

 a later period of this instar (Bodenstein, 

 '41). The genital discs of last instar Droso- 

 phila larvae also still possess considerable 

 powers of regulation (Hadorn, Bertani, and 

 Gallera, '49). At this advanced stage these 

 discs are composed of a mosaic of separate 

 fields, each of which gives rise to a special 

 element of the imaginal genital apparatus 

 These fields cannot replace each other, but 

 the various parts within a single field can 

 regulate; they differentiate into normallv 

 shaped adult structures of characteristic size 

 Duplications of various other Drosophila or- 

 gans, such as wing, antenna, palpus, o^ 

 scutellum. following x-ray treatment (Wad- 

 dington, '42) or temperature treatment (Vogt. 

 '47a) of voung last instar larvae gave further 

 proof of the indeterminate state of these or- 

 gans in late stages of development. 



The determination of all imaginal systems 

 is not necessarilv completed at the end of the 

 larval period. The venation in wings o^ 

 come Lepidoptera is not finally determined 

 before the pupa is several days old (Henke, 

 '33). It is also during the pupal stage tha+ 

 the color pattern of the wing becomes deter- 

 mined. This wing pattern is a comnlicated 

 developmental svstem, composed of different 

 zones which are determined at different 

 times. In the determination of the wing 



