Descriptive Biology 



97 



A still further stage in sex development is shown by a 

 distinct difference in size and activity of the copulating cells. 

 The malaria organism multiplies asexually in the blood cells 

 of its host. After a time, under conditions not well under- 

 stood, some of the malarial cells enlarge. If now the patient 

 is bitten by one of the Anopheles mosquitoes, which transmit 

 the disease, some of these enlarged cells remain quiescent, 

 forming the female cells in the mosquito's stomach, while 

 others cast off a number of small active filaments or male 

 cells. These latter then unite with or fertilize the former, 



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Life Cycle of the Malarial Organism 



a, parasite in red blood corpuscle; b and c, spore formation; d, female, 

 and e, male cells, which are uniting at f ; g, sporozoites in cyst; h, sporo- 

 zoite free; i, ameboid parasite developed from h, prepared to enter red 

 blood corpuscle, j. Original. 



and from their union a large number of minute motile cells 

 or "sporozoites" are formed, by which the asexual cycle is 

 repeated when the infected mosquito bites a new victim. 



Yet a further and final step in sex differentiation among 

 the unicellular forms is found in Volvox, an organism which 

 is on the fence, so to speak, between Protozoa and Protophyta ; 

 and which forms a bone of contention between the botanists 

 and zoologists, each claiming ownership to it. Volvox is a 

 hollow spherical group of cells, numbering in some cases over 

 20,000, and reaching a diameter of 1/25 of an inch. The cells 

 carry a pair of cilia each, by means of which the organism is 



