540 



GENERAL PHYSIOLOGY 



theless, from all these individual differentiations atoms and groups 

 of atoms pass out continually in definite directions, and into them 

 new atoms and molecules enter continually, so that the structure 

 is continually being destroyed and rebuilt. There is, therefore, a 

 continual stream of matter which ramifies into the various differ- 

 entiations in an extremely complicated way and possesses a very 

 different composition in its different parts. This stream of matter 

 is the expression of the complex metabolic relations between the 

 individual parts of the cell-body, and it is the direct condition of 

 the very definite and peculiar form of the cell in question. The 

 structure is able constantly to re-establish and maintain itself, only 

 when certain atoms are at the necessary place at the right time. 

 If the stream of matter ceases, the molecules disintegrate, and the 

 definite grouping is dissolved. So long, however, as the stream of 



matter is uninterrupted, the individual 

 molecules and atoms take up by attraction 

 the necessary particles, and the structure 

 continues to exist. If the stream of matter 

 changes in the direction and composition 

 of its particles, the form of the cell and 

 its differentiations must change also, and 

 there is a development. 



We have already found the comparison 

 of vital phenomena with a flame very per- 

 tinent in many respects. This simile is 

 also adapted to make clear in an especially 

 striking manner, the relation between 

 form-construction and metabolism. The 

 butterfly figure of a gas-flame has a very 

 characteristic differentiation of form. At 

 the bottom immediately above the slit in 

 the burner, there is complete darkness; 

 above it there is a blue zone, only 



feebly luminous ; and above that upon each side the bright 

 luminous surface is extended out on both sides like the wings of 

 a butterfly. This peculiar form with its characteristic differentia- 

 tions, which continues to exist so long as the position of the 

 gas-cock and the surrounding conditions are not changed, depends 

 solely upon the fact that in the individual parts of the flame, the 

 grouping of the molecules of illuminating gas and oxygen is very 

 definite, although the molecules themselves change at every 

 interval. At the bottom of the flame the molecules of illuminat- 

 ing gas are pressed together so closely that the oxygen necessary 

 for their combustion cannot come in between them, and as a 

 result of this there is here complete darkness. In the bluish 

 zone some molecules of oxygen are combined with those of 

 illuminating gas, and the result is a feeble light. In the large 



FIG. 263. Poteriodendron. A 

 single individual of a colony. 

 The cell-body, fastened upon a 

 myoid-fibre, sits in a bell- 

 shaped cup and contracts its 

 flagellum. 



