72 BIOLOGY 



body penetrates the egg, the nucleus it contains unites with the 

 nucleus of the egg, shown at k and I. After this union the com- 

 bined mass grows rapidly in size, / to o, and eventually breaks 

 up into an immense number of minute spores, p, greatly in 

 excess of those found at the stage g in human blood. These 

 minute spores lodge in the salivary glands of the mosquito, and 

 are ejected into the blood of the person bitten by the mosquito. 

 Thus a new human individual is inoculated with the spores, 

 which find their way into the blood corpuscles of the new victim 

 and produce the disease. It is not the most common mosquito 

 (Culex) that is concerned in this history, but one that is ordi- 

 narily less abundant, a species called Anopheles. From these 

 facts it follows that malaria will not occur in any locality unless 

 this particular mosquito is present; and further, that only the 

 mosquitoes which have previously bitten malarial patients will 

 be able to carry the infection. 



It will thus be seen that the malarial organism passes through 

 two stages in its life cycle, reproducing itself in each by the 

 production of spores, though the spores are of two different 

 kinds; and that at one stage there is a union of cells of unequal 

 size, which may probably be regarded as a true sex union. All 

 stages of its life are passed within the bodies of other animals, 

 and it is thus wholly parasitic. The three different species of 

 the malarial organism have similar life cycles, though differing 

 slightly in details. 



The malarial organism passes through two stages, in its life 

 cycle, each in different animals. Such a complicated history, 

 in which there is more than one distinct stage, is known as a 

 metamorphosis (Gr. meta = beyond + morphe = form). Many 

 other animals have a metamorphosis, one of the best-known 

 examples being that of the butterfly, which passes through 

 the well-known states of egg, caterpillar, cocoon, and butterfly. 

 Another example is the frog; see page 286. A metamorphosis 

 is thus found both among higher animals and also among the 

 lowest. 



