66 GENERAL PHYSIOLOGY 



Schultze's definition, however, does not include all the essential 

 or general cell-constituents. In 1833 Brown had discovered in 

 protoplasm a further specific structure, the cell-nucleus, which,_ by 

 its refractive power, could be distinguished clearly as a spherical 

 granule from the protoplasm enclosing it. Schleiden ('38) took 

 up this discovery of Brown and demonstrated the cell-nucleus to 

 be a wide-spread constituent of the cell in many plants ; but he 

 was misled in his theory of jjhytogenesis into considering the 

 nucleus as the element from which the cell first arises in the 

 course of the individual development of the plant. Since that 

 time constantly more attention has been given to the nucleus. 

 It was found not only in plant-cells, but after Schwann's labours 

 ('39) also in the most diverse animal cells. But, especiall}' 

 when by means of certain colouring-matters, such as carmine, 

 hsematoxylin, etc., it was stained, and thus was made clearly 

 visible in the protoplasm in which it was embedded, the view was 

 gradually adopted that it represents a very characteristic con- 

 stituent of the cell ; and soon the question arose whether cells ever 

 exist without nuclei, or whether the nucleus is a general and, like 

 the protoplasm, essential constituent of the cell. 



Among the unicellular free-living Rhizopoda, to which Max 

 Schultze had called attention, Haeckel ('70) found a considerable 

 number in which no trace of a nucleus was to be demonstrated, 

 and which, since they appeared to consist of a simple bit of proto- 

 plasm and thus were the lowest and simplest conceivable organisms, 

 he termed Monera. Another group in which no nucleus could be 

 demonstrated was that of the micro-organisms, the Bacteria, which 

 likewise have excited the greatest interest in recent times. Thev 

 are the smallest of all existing living beings, and, although they 

 possess a fixed unchangeable form, they reveal no trace of dif- 

 ferentiation in their apparently wholly uniform protoplasmic 

 bodies. If we except the red blood-corpuscles of warm-blooded 

 animals, which likewise show no differentiation of their bodv- 

 substance into two separate parts, protoplasm and nucleus, but 

 which, as has been demonstrated, develop from actual nucleated 

 cells, the two groups of the Monera and the Bacteria remain as the 

 sole apparently non-nucleated cells. 



But with the recent wonderful development of the technique 

 of microscopic staining the conception of the Monera as non- 

 nucleated cells has gradually changed. By the employment of 

 the newer, complicated staining-methods constantly more of 

 the organisms which Haeckel described as Monera are being 

 recognised as nucleated cells : in many of them even a large 

 number of small nuclei have been demonstrated ; and Gruber 

 ('88) has found forms in which the nuclear substance is 

 distributed through the whole protoplasm in innumerable, 

 extremely minute granules (Fig. 8). Thus the number of the 



