DARK-GROUND ILLUMINATION 353 



i. The small particles with Brownian movement generally 

 seen in favourable cases showed the protoplasmic colloid to be 

 of the nature of a hydrosol. 



2. These particles can unite with one another, forming aggre- 

 gates ; or break up, thus decreasing or increasing in number. 

 (This may be related to variations in the general vitality or 

 nutritive condition of the cell. 1 ) 



3. In other cases, cells which were undoubtedly living, and 

 generally speaking well nourished, failed to show any such 

 movement, but the motion may have been masked by the small- 

 ness and close proximity of the particles. 



4. The spontaneous change from the sol state to the gel or 

 vice versa was not observed in the living cell. On the death of 

 a cell, however, complete coagulation of the sol takes place, with 

 cessation of the Brownian movement, while the gel thus formed 

 gives an appearance of crowded diffraction images under dark- 

 ground illumination. 



5. The colloid complex of the protoplasm consists of a 

 reversible and an irreversible portion. 2 This is deduced from 

 the behaviour of broken living and dead cells in water. Some 

 particles produce a colloid solution with the water — the rever- 

 sible portion — while others aggregate and remain together — the 

 irreversible portion. 



6. Since the protoplasm contains an irreversible colloid, the 

 taking up of an electrolyte by the cell should result in its 

 coagulation. Some evidence is brought forward to show this, 

 but the matter requires further investigation. 



It may perhaps be said that the method has not realised to 

 the full, the expectations of those who hoped it would clear up 

 definitely certain vexed questions of cell structure. The idea of 

 the method generally suggests the possibility of its application 

 to the cytological study of the nucleus and the behaviour of the 

 chromosomes in the living nucleus. In this direction but little 

 help has been derived from the method up to the present, and 

 only in a few cases has nuclear structure been observed. The 

 difficulty of choosing suitable material is even greater than ever, 

 and generally only resting nuclei have been observed. Where 

 this has been done, the nucleus seems to show little except the 



1 v. Bechold, I.e. p. 256. 



2 See any work on colloids, e.g. Introduction to Physics and Chemistry of 

 Colloids, Emil Hatschek. (T. & A. Churchill, 1913, 2s. 6d.) 



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