18 



GENERAL SKETCH OF THE CELL 



showed, however, that most living cells are not hollow but solid 

 bodies, and that in many cases for example, the colourless corpuscles 

 of blood and lymph they are naked masses of protoplasm not sur- 

 rounded by definite walls. Thus it was proved that neither the 

 vesicular form nor the presence of surrounding walls is an essential 

 character, and that the cell-contents, i.e. the protoplasm, must be the 

 seat of vital activity. 



Within the protoplasm (Figs. 6-8) lies a body, usually of definite 

 rounded form, known as the nucleus^ and this in turn often contains 



Attraction-cph-re enclosing two cer.trosomes. 



f Plasmosome or 



true 



nucleolus 



Nucleus > 



Chromatin- 



network 



Linin-network 



Karyosome, 

 net-knot, or 

 chromatin- 

 nucleolus 



Plastids lying in the 

 cytoplasm 



Vacuole 



Passive bodies (meta- 

 plasm or paraplasm) 

 suspended in the cy- 

 toplasmic meshwork 



Fig. 6. Diagram of a cell. Its basis consists of a meshwork containing numerous minute 

 granules (microsomes) and traversing a transparent ground-substance. 



one or more smaller bodies or nncleoli. By some of the earlier 

 workers the nucleus was supposed to be, like the cell-wall, of sec- 

 ondary importance, and many forms of cells were described, as being 

 devoid of a nucleus ("cytodes" of Haeckel). Nearly all later re- 

 searches have indicated, however, that the characteristic nuclear 

 material, whether forming a single body or scattered in smaller 

 masses, is always present, and that it plays an essential part in the 

 life of the cell. 



Besides the presence of protoplasm and nucleus, no other struc- 

 tural features of the cell are yet known to be of universal occurrence. 



1 First described by Fontana in 1781, and recognized as a normal element of the cell by 

 Robert Brown in 1833. 



