PHYSIOLOGICAL 



367 



in those animals that have well-developed nervous and vascular 

 systems, and circulating chemical messengers or "hormones" which 

 contribute greatly to the harmonious regulation of the whole Hfe. 

 Third, there is the embryological statement, that the individual 

 many-celled organism begins its life, in all ordinary cases, as a 

 fertilised egg-cell, which divides and re-divides to form an embryo. 

 In other words, developing and growing imply cell-division. Here 

 again there is some need of a saving clause, since it is not enough 

 to picture the body being built up by adding cell to cell, as a house 

 by adding brick to brick. There is an important idea in De Bary's 

 caution: "It is not that the cells make the plant; it is the plant 

 that makes the cells." For the cellular structure is an arrangement 



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Fig. 48. 



Diagram of a Typical Cell. PL, plastid; CC, centrosome; n, nucleolus; N, 

 nucleus; CHR, chromosome; CT, cytoplasm; V, vacuole; GR, granule, 



which makes it possible to have a body with much division of 

 labour. Cellular structure is a condition of differentiation. 



But after the formulation of the Cell Doctrine, it gradually 

 became evident that the structure of the unit was complex to an 

 unforeseen and extraordinary degree, just as the atom has slowly 

 revealed the complexity of its organisation in modern physics. 

 The main conclusions of Schwann and Schleiden remain true to-day, 

 but the picture of the cell has become much more intricate. 



Protoplasm and its Study. — In the early days of the Cell 

 Doctrine more attention was paid to the cells than to their contents. 

 The chemical aspect of life was but slowly appreciated, and what is 

 obvious to every student to-day was not realised in 1840, that vital 

 activity is essentially bound up with chemical and physical changes 

 in the viscid matter that is present in almost all cells. It was largely 

 through the study of unicellular organisms, like Amoebae and Fora- 

 minifera, that the idea became clear that the kind of activity we 

 caU "life" was the outcome of changes in the contents of the cells. 



