DIFFERENTIATION OF STRUCTURE 19 



damp walls, &c. For comparison we may also study 

 a single cell from the human body, a white blood 

 corpuscle or, as it is sometimes termed, a leucocyte. 



The animal Amoeba, under a high magnifying Amoeba, 

 power, appears as a minute mass of protoplasm, 

 granular in the centre and clearer towards the 

 margin and provided with a nucleus. It creeps slowly 

 over the substratum by sending out projections 

 from its margin, technically called pseudopodia. 

 Should it meet, in its progress, particles of organic 

 substances suitable for food, it flows slowly round 

 them and engulfs them. In its interior all that is 



A >W B 



FIG. 9. A, Pleurococcus ; B, Chlamydomonas. X 450. 



nutritively serviceable is digested and absorbed, 

 and the remainder is rejected. 



A leucocyte from the blood is, to all intents and 

 purposes, a small Amoeba. Being constantly bathed 

 in a nutrient fluid, viz., the blood, it is in a particu- Leuco- 

 larly favourable situation for obtaining fluid nourish- cyte- 

 ment ; still, it has been found that should extraneous 

 bodies detrimental to the human organism find their 

 way into the blood, the leucocyte is capable of 

 engulfing and destroying them. The parallel with 

 Amoeba is thus fully maintained. 



The third type of unicellular i organism we have 

 chosen is, however, different in many respects 

 from those already mentioned. In the first place, pi eu ro- 

 Pleurococcus (Fig. 9A) although it also consists coccus. 



