90 ZOOLOGY 



the cell organs so that rapidly the homologies of permanent 

 constituents of the cell could be agreed upon and definite standard 

 tests of structure and behaviour fixed. 



It would be impossible at this point to omit mentioning the 

 advent of two valuable instruments which have come into the 

 hands of the animal microscopist. Firstly, the perfection of an 

 instrument f'or micro-dissection by means of which living cells and 

 organisms can be manipulated has relieved the microscopist of the 

 temptation to guess at the physical nature of the cell constituents. 

 Paraffin sections and smear preparations are no longer the only 

 means by which he can examine cells. A very considerable body 

 of evidence has been collected by this means, so that the physical 

 picture which a cytologist should call up in his mind of the cell 

 structures need no longer be based on staining and fixing capacity 

 of the particular structure or the technique applied. In this 

 direction investigation has only just started and much more is to 

 be expected from it. 



Secondly, the technique of tissue cultivation in vitro has 

 contributed much to the microscopist's power. The growth of 

 tissue in a layer one cell thick has made it possible to follow cell 

 behaviour continuously instead of by a series of pictures compiled 

 from sections and arranged in (possibly) the right order. Both 

 this and the preceding technique suffer from limitations chiefly 

 optical in nature, but the use of the ultramicroscope opens up 

 possibilities which have only been tentatively explored. 



The animal cell can be divided into two parts, the nucleoplasm 

 (karyosome) and cytoplasm (cytosome). The nucleoplasm consists 

 principally of the true nucleus which is composed of chromosomal 

 material. There is also present, however, the nucleolus in the form 

 of a sphere (sometimes irregular in shape). This is frequently 

 termed the plasmosome when definite activity is observed in it. 

 The nucleolus appears to be quite distinct from the chromatin of 

 the true nucleus, although many of its staining reactions are 

 similar. Its functions are such that it is usually considered at the 

 same time as various inclusions in the cytoplasm, to which it 

 appears to be functionally related. 



In the cytoplasm there are present a number of inclusions. 



