24 NATURAL HISTORY OF OUR SHORES 



this envelope is absent, and the cell has, at most, a boundary 

 of the same as the contents, only of rather firmer nature. 



The contents of the cell are, as I have already said a 

 glairy semi-fluid substance termed protoplasm, but it is not 

 homogeneous all through, but is arranged in regular system. 



In one part of the cell is a little body termed the nucleus, 

 and within this, or adherent to it, a smaller body termed 

 a nudeolus. There is also a space, that appears hollow, but 

 which contains a watery fluid, termed the vacuole ; further, 

 a very delicate network permeates the whole structure. 



The lowest organisms, as we have seen, consist of one 

 cell only, and its division is with these not growth, but 

 multiplication. 



In the vegetable kingdom a good example of these one- 

 celled organisms is seen in Protococcus, the minute plants 

 that appear as a powdery green coating to the trunks of 

 trees, palings, etc., in sheltered situations. 



When a cell is about to divide the process commences 

 inside, the nucleus being the first part to divide. As its 

 halves are drawn apart a series of fibres follow the divided 

 portions in a regular pattern, which microscopists term 

 Jcaryokinetic lines. Then the cell becomes constricted from 

 without, and becomes two. 



Some cells have non- retractile filaments, which are termed 

 cilia when there are many, or flagellce when there is but one 

 to each cell. 



This is a very bare outline of cell structure, but it will 

 serve our present purpose. 



In sponges there are, besides the round cells which form 

 the greater part of the mass, others which have a cup- 

 shaped collar, and within this a flagellum. These are 

 termed " flagellated cells " ; and an " egg " is in the sponge, 

 as in every other living thing, a specially modified cell. 



After this little excursion into the fields of biology we 

 shall get back to the more congenial task of observation 



