THE GERM-PLASM THEORY 



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Oi ? 



Uij 



1112 





skin-layer of it, the < hypodermis,' wl,i,l,. however in thi 

 undoub edl, 'determines .the form. But the in ternai P ar£ 1 1 



outsMe T °H Gr T UPS ^ - rmV bto the i-P-J ^ " 



outside. Something similar probably takes place m the ca , all 



organs which are made up of many parts; they are, bo to speak, drt 



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



determinants are brought into co-operation wl, dative ralue in 



determining the form and function of the organ may be rery diverse 

 m For it is undoubtedly a very different matter whethera cell 



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



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



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 or other 



substances within itself if these 



be presented to it. Between 



these two extremes of determining 

 power there are many inter- 

 mediate stages. Cells which con- 

 tain the determinants of blood- 

 vessels, tracheae, or nerves need 

 not be so definitely determined 

 that they always give rise to 



precisely the same blood-vessels, the same branching of the trachea 

 the same bifurcation of nerves; they may probably posi — no more 

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

 special form taken by the nerves, tracheae, <>r blood-vessels may 

 essentially determined by their environment. Thus in the morbid 

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

 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 Bhort, 

 the cells were only determined to this extent, thai 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 eon- 



Fi<;. 89. Anterior region "f th< 



oi'a Mid-- < 



Tk, thorax. ui } inferior imagina] <li- 

 oi, superior imagina] disks. 

 ami /'/ : . the primordia of the lie 

 oi - and oi :: . the primordia of th< 

 and ; balancers. 1 gr, brain. ; r, i 

 ventral ganglia with nerves whicl 

 the Lmaginal disks, fro, tracheal t 

 Enlarged aboul 15 I inn 



