74 LECTURES ON BIOLOGY 



distributing it throughout the body. Sometimes, however, when 

 morbid changes of the organism facilitate an excessive increase 

 in the number of leucocytes they may become a serious danger. 

 They are then no longer content to feed on bacteria and senile 

 body-cells which are no longer able to do useful work, but make 

 raids upon the healthy tissue. In contrast to the phagocytes, 

 most of the other body-cells are unable to absorb solid par- 

 ticles but are restricted to fluid food. 



If we observe an Amoeba under the microscope we might 

 easily be led to conclude from the fact that the changes take place 

 only in the protoplasm that only this substance is essential 

 to the course of the functions of life. This would, however, be 

 wrong, for though invisible it is the cell-nucleus which regulates 

 the various processes. Only when nucleus and protoplasm co- 

 operate normal life becomes possible. 



I shall here only be able to give a very general description 

 of the nucleus. Like the cell itself, it is greatly variable in size. 

 As a rule, its size is proportioned to the amount of surrounding 

 protoplasm. Thus we find in the large ganglion cells, and in 

 particular in immature ova, nuclei of considerable dimensions, 

 which we can observe with the naked eye and take out of the 

 cell-body with a pin. But this rule is not without exception, for 

 in ripe and fertilized ova the nucleus becomes sometimes so 

 minute that it is difficult to demonstrate its presence. The 

 normal position of the nucleus is in the centre of the cell, but 

 sometimes it leaves its place and wanders to other positions. 

 But wherever it may be, it remains always surrounded by proto- 

 plasm. According to their behaviour towards certain stains we 

 distinguish in the nucleus two main substances, of which one, 

 chromatin, exhibits a strong inclination towards dyes such as 

 haematoxylin, carmine and others, and stains intensively, whilst 

 the other, achromatin or limn, does not as a rule stain. 



In its morphology the nucleus repeats on a minute scale the 

 structure of the whole cell (see fig. 20). Here, too, the main 

 substance is a light-coloured, filamentous fluid which is known as 

 the nuclear juice. Its volume mainly decides the size of the 

 nucleus. Embedded in the nuclear fluid is a very fine mesh- 

 work formed of linin-threads which according to many investi- 

 gators must be regarded as the optical expression of the alveolar 



