LESSON VI 



A COMPARISON OF THE FOREGOING ORGANISMS WITH CER- 

 TAIN CONSTITUENT PARTS OF THE HIGHER ANIMALS 

 AND PLANTS 



When a drop of the blood of a crayfish, lobster, or crab is 

 examined under a high power, it is found to consist of a 

 nearly colourless fluid, the plasma, in which float a number 

 of minute solid bodies, the blood-corpuscles or leucocytes. 

 Each oi these (Fig. 8, a) is a colourless mass of proto- 

 plasm, reminding one at once of an Amoeba, and on 

 careful watching the resemblance becomes closer still, for 

 the corpuscle is seen to put out and withdraw pseudopods 

 (a^ — A*) and so gradually to alter its form completely. 

 Moreover the addition of iodine, logwood, or any other 

 suitable colouring matter reveals the presence of a large 

 nucleus (a^, a^, nu) : so that, save for the absence of a con- 

 tractile vacuole in the leucocyte, the description of Amoeba 

 in Lesson I. would apply almost equally well to it. 



The blood of a fish, a frog (b^), a reptile, or a bird contains 

 quite similar leucocytes, but in addition there are found in 

 the blood of these red-blooded animals bodies called red 

 corpuscles. They are flat oval discs of protoplasm (b"', b^) 



