ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



inquire as to the composition of the remains of the protoplasm which 

 are left when the life has departed from it. Analysis shows that these 

 consist of a mixture of those complicated substances known to the 

 chemist as proteins — compounds of Carbon, Oxygen, Hydrogen, Nitrogen 

 and Sulphur in about the following percentages : C52, O23, H7, N16, S2. 

 Chemists are not yet able to tell us precisely how the atoms of these 

 various elements are united to form the very complex molecules of the 

 various kinds of proteins. 



The student of living things is, for the reason indicated above, to a 

 great extent debarred from using what to the Chemist or Physicist is 



his most powerful instrument of in- 

 vestigation, the method of analysis, 

 whereby the complicated subject 

 of investigation is split up into 

 its simpler components and these 

 studied individually. He is therefore 

 driven to make his main stand-by 

 the method of mere observation- — in 

 the case of small creatures like Amoeba 

 with the aid of the microscope. 



When the living Amoeba is 



observed under the microscope it is 



seen (Fig. i) to form an irregularly 



shaped blob of protoplasm — greyish 



white when seen against a dark 



background, clouded and finely 



granular when seen against a light 



background. The irregularity of 



form is characteristic and no two 



specimens are exactly alike. 



Examination under a high magnification shows that the blob of 



protoplasm is not homogeneous but consists of a main larger portion 



known as the cytoplasm, and embedded in this a rounded denser portion 



— the nucleus (Fig. i, «). 



The cytoplasm is further seen to consist of a main portion known as 

 the endoplasm (Fig. i, en) and a thin superficial layer — the ectoplasm 

 (Fig. I, ec). Of these the endoplasm is fluid in its nature and is laden 

 with minute particles which give it its very characteristic granular 

 appearance. " By the use of very high magnifications and the application 

 of appropriate tests it can be determined that these granules differ 

 in character. Some are droplets of fat, some are crystals of waste 



Fig. i. 



Amoeba proteus. c.v, Contractile vacuole ; 

 cc, ectoplasm j en, endoplasm ; f.v, food-vaC' 

 uole ; «, nucleus ; ps, pseudopodium. 



