THE PROTOZOA-THE DAWN OF LIFE 23 



starts one of these colonies divides, and the products of division 

 do not separate, but remain connected as a loose body of numer- 

 ous cells. In this hollow spherical cluster of cells, some are 

 set apart and eventually liberated as reproductive cells which 

 will start new colonies ; while other cells surrender their power 

 of fission so inherent in the Protist cell, becoming vegetative cells, 

 the supply of food material due to their living activities going 

 to the nourishment of the reproductive cells. It is these vegetative 

 cells which perish when the ruptured sphere sinks down into the 

 mud on the floor of the pond. 



Under certain conditions the reproductive cells may give rise 

 to two distinct types of cell ; one small, yellow in colour, rod-like, 

 and with two flagella ; the other an enlargement of the original 

 cell, which has become enriched at the expense of neighbouring 

 vegetative cells. Conjugation takes place between these dis- 

 tinctive cells ; the flagellate forms escaping, swim about freely, 

 and conjugate with the large motionless cells to form a zygote, 

 which, after a period of rest, divides and gives rise to a new 

 colony. 1 



Here, then, we find a differentiation of reproductive cells in 

 an active flagellate male cell, a passive richly stored female cell, 

 each functioning in the same manner as the active and passive 

 male and female reproductive cells of the higher animals, and by 

 their union or conjugation producing a similar result. 



Of recent years the Haemoflagellata or Trypanosomes 

 have attracted a great deal of attention, on account of the appal- 

 ling destruction of human life from diseases set up by the pre- 

 sence of these organisms in the blood system. Like the malarial 

 parasite already described, these Haemoflagellates are transmitted 

 to man through the agency of certain blood-sucking insects, but 

 the disease set up, on account of its peculiarly fatal character, is 

 far more appalling than malaria. Sleeping sickness, or human 

 trypanosomiasis, as this terrible disease is called, appears to have 

 existed among the natives of the West African coast from the 

 remote past, and to occur, though rarely as a very serious scourge, 

 throughout tropical West Africa. But from there it has crept 



1 When two individual cells come together and become completely fused, the 

 process is known as Conjugation, the body formed by the union of the cells being 

 known as a Zygote. 



