14 OUTLINES OF EVOLUTIONARY BIOLOGY 



The nucleus (Fig. 2, nu.), composed of a somewhat different 

 variety of protoplasm sometimes known as karyoplasm or 

 nucleoplasm, is a very definite body of more or less rounded 

 form, sometimes shaped like a bun, and easily distinguishable 

 from the cytoplasm even in the living animal by its more highly 

 refractive character. Its position is by no means constant, for 

 it floats about from place to place in the interior ofjthe almost 

 liquid cell-body. 



The cytoplasm is very imperfectly differentiated into infeer and 

 outer portions. The former, in which the nucleus is lodged, is 

 often called the endosarc or endoplasm (Fig. 2, end.), the latter 

 the ectosarc or ectoplasm (Fig. 2, ect.). The ectoplasm must be 

 regarded as a feebly developed protective layer ; it is the part which 

 comes into direct relation with the surrounding water and through 

 which all intercourse between the Amosba and its environment 

 must take place. Though soft and gelatinous, it is a good deal 

 firmer and denser than the endoplasm, and &4s-atstr-more trans- 

 parent, for the endoplasm contains imbedded in it numerous more 

 or less opaque particles of various kinds which give it a coarsely 

 granular character. Most of these particles are minute, but 

 others are generally present of comparativel^Jarga size~ and 

 enclosed in drops of clear liquid. These latter are food particles 

 (Fig. 2, f.p.) undergoing digestion, and they can frequently be 

 identified as the bodies of other organisms which the Amosba 

 has taken in, microscopic plants or animals smaller than itself. 

 The drops of liquid in which they occur are termed foo$ 

 vacuoles (Fig. 2, /.#.). 



Another spherical drop of liquid (Fig. 2, c.v.) may be observed 

 somewhere near the surface of the cell-body. This is perfectly 

 clear and contains no solid particles ; moreover it undergoes a 

 rhythmical dilatation and contraction, gradually increasing to a 

 maximum size and then suddenly disappearing owing to the 

 discharge of its contents into the surrounding water. If the 

 spot where this " contractile vacuole " disappears be carefully 

 watched another drop of liquid is seen gradually to accumulate 

 there, and the process is repeated. 



We are told that in the early days of chemistry, before the 

 highly specialized apparatus which is now used was thought of, 

 the originator of the atomic theory performed his experiments 

 with the ordinary domestic crockery. So also an Amosba is able 

 to perform, with the extremely simple apparatus at its disposal, 



