342 



Embryogenesis: Progressive Differentiation 



efferentia from species with spiral testes are 

 brought experimentally into contact with 

 normally non-spiral testes, the latter coil, 

 i.e., become spiral shaped. And vice versa, 

 the vasa of non-spiral species when con- 

 nected to normally spiral testes prevent the 

 coiling of these organs. The vasa thus de- 

 termine the final shape of the adult testis. 

 This effect is only produced when testes and 

 vasa, which originate independently of each 

 other, become attached, i.e., come into cel- 

 lular contact. The attachment of both organs 

 takes place during pupal life, at which time 

 both components involved are already quite 

 well differentiated. 



Another type of interaction between tis- 

 sues far advanced in development is illus- 

 trated in the following example. If female 

 genital discs of Drosophila are transplanted 

 into male hosts, the host testes suffer exten- 

 sive degeneration when the oviducts devel- 

 oped from the transplanted disc become at- 

 tached to the host testes. As in the previous 

 case, the phenomenon is only brought about 

 if cellular contact is established between 

 these two organs; the principle causing it is 

 not species specific (Bodenstein, '46). In both 

 cases cited, the stimuli eliciting the charac- 

 teristic response are located in the genital 

 ducts. There seems to exist an inductor-re- 

 actor relationship between these two organs; 

 the inductor in the vasa calls forth a growth 

 reaction in the testis, and the inductor in the 

 oviduct a degenerative process. The inductors 

 act only when in contact with the reacting 

 material. 



A similar mechanism of transmitting deter- 

 mining influences through the substance of 

 adjacent cells has been suggested in explain- 

 ing the determination of certain structural 

 imits in the cuticle pattern of the bug 

 Rhodnius (Wigglesworth, '40a). In this insect, 

 the nymphal cuticle bears bristles arising 

 from small plaques which are distributed 

 regularly over the surface of the abdomen. 

 The closeness of these plaques to each other 

 seems to affect the determination of new 

 plaques. Each plaque apparently exerts 

 through the substance of the epidermis cells 

 that surround it an inhibiting influence 

 which prevents the development of a new- 

 plaque within a certain radius. The distance 

 over which this influence acts is dependent 

 upon the number of epidermal cells between 

 the existing plaques. It is noteworthy that the 

 number of cells intervening between the 

 plaques decides the limitation of the inhibit- 

 ing influence. Since the cell number is the 

 result of previous cell division, the role of 



growth processes in the determination events 

 again becomes evident. In the mechanism of 

 determination, influences of this kind are per- 

 haps of major importance, although they are 

 not easily detected experimentally. They 

 might function fundamentally in the develop- 

 mental interactions between cell populations, 

 prospective organ regions, discs or body parts 

 discussed in this chapter. 



HORMONES IN GROWTH AND 

 DIFFERENTIATION 



While the processes of determination es- 

 tablish, by progressive restriction of develop- 

 mental potencies, the determination of organs 

 and body parts to attain fixed fates, it is 

 hormones that in many instances are known 

 to be responsible for the realization of these 

 fates. Hormone activity thus is an integral 

 mechanism in the development of insects. 

 For instance, the imaginal differentiation of 

 Drosophila leg or eye discs can only take 

 place after a specific hormone released by 

 the ring gland has exerted its influence 

 (Bodenstein, '43). Similarly, the nymphal 

 cuticle of the blood-sucking bug Rhodnius 

 (Wigglesworth, '34 and '36) or of the cock- 

 roach (Scharrer, '46) or the skin of cater- 

 pillars (Piepho, '38a) can only develop 

 imaginal character when stimulated by the 

 appropriate hormones. Moulting in insects 

 is also under humoral control. Since moult- 

 ing is an essential feature in insect growth, it 

 is growth that is controlled by hormones. 

 This fact is also well illustrated by the be- 

 havior of Drosophila discs which grow only 

 in an organic environment especially condi- 

 tioned by hormones (Bodenstein, '43). The 

 ability of the different tissues to react to 

 hormonal stimuli varies greatly. Some tissues 

 become competent for response early in 

 development, others later. Thus the epidermis 

 of first stage Rhodnius nymphs (Wiggles- 

 worth, '34) or of caterpillars just hatched 

 from the egg (Piepho, '38b) already responds 

 at this early stage to metamorphosis hor- 

 mones and can hence be changed experi- 

 mentally into imaginal epidermis. On the 

 other hand, imaginal discs of first instar 

 Drosophila larvae do not respond to these 

 hormones bvit acquire the ability to do so at 

 the next larval stage (Bodenstein, '44). 

 Moreover, not all tissues respond with like 

 ease to the same hormone level (Bodenstein, 

 '43; Klihn and Piepho, '36). The factors that 

 make the discs or body parts competent to 

 respond, or what this competence means in 

 physicochemical terms, is obscure, as is the 



