THE GERM-PLASM THEOEY 



393 



Th\\ ^'3 



trb 



skin-layer of it, the ' hypodermis,' wliicli, however, in this case 

 undoubtedly determines the form. But the internal parts of the leg, 

 especially the nerves, tracheae, and probably also the muscles, are 

 formed from other cell-groups and grow into the imaginal disk from 

 outside. Something similar probably takes place in the case of all 

 organs which are made up of many parts ; they are, so to speak, shot 

 together from several points of origin, from various primordia; and 

 determinants are brought into co-operation whose relative value in 

 determining the form and function of the organ may be very diverse. 

 For it is undoubtedly a very different matter whether a cell bears 

 within it the elements which compel it in the course of growth to 

 develop an organ, for instance a leg, of quite definite size, sculpture, 

 number of joints, and so on, or 

 whether it only bears the somewhat 

 vague power of determining that 

 connective tissue or fatty tissue is 

 to be produced. In the first case 

 it controls the whole formation of 

 the part, in the second it only fills 

 up gaps or lays down fat ox other 

 substances within itself if these 

 be presented to it. Between 

 these two extremes of determining- 

 power there ai^e many inter- 

 mediate stages. Cells which con- 

 tain the determinants of blood- 

 vessels, trachea3, or nerves need 

 not be so definitely determined 

 that they always give rise to 



precisely the same blood-vessels, the same branching of the tracheae, or 

 the same bifurcation of nerves ; they may probably possess no more' 

 than the general tendency to the formation of such parts, and the 

 special form taken by the nerves, tracheae, or blood-vessels may be 

 essentially determined by their environment.. Thus in the morbid 

 tumours of Man, nerves, and especially blood-vessels, may develop hi 

 a quite characteristic manner, which was certainly not determined in 

 advance, but has been called forth by the stimulus, the pressure, 

 and other influences of the cellular basis of the tumour. In short, 

 the cells were only determined to this extent, that they con- 

 tained the tendency to give rise to blood-vessels under particular 



influences. 



It would be a mistake, however, to think of the primary con- 



Fig. 89. Anterior region of the larva 

 of a Midge (Corethraplumicornis). K, head. 

 Th, thorax. iti, inferior imaginal disks. 

 oi, superior imaginal disks, ui^, ui"^, 

 and ui^, the primordia of the limbs. 

 oi ^ and oi ^, the primordia of the wings 

 and ' balancers.' g, brain, bg, chain of 

 ventral ganglia with nerves which enter 

 the imaginal disks, irb, ti'acheal vesicle. 

 Enlarged about 15 times. 



