234 LECTUEES ON BIOLOGY 



germ-cells chiefly by a great reduction of their protoplasm, in 

 the egg-cells by a very ingenious means with which I will deal 

 in the next lecture. 



We see, further, that the separation of the organic world into 

 two sexes, the existence of male and female sex-cells is no original 

 state but a secondary acquisition which has been developed very 

 gradually by adaptation to changed external conditions of life, 

 and hand in hand with the differentiations in organization. 

 Division of labour, this omnipotent factor which rules the entire 

 organic world, is here again the motive power to which we are 

 able to trace all phenomena. 



Apart from reproduction by fission we know in many 

 infusorian s and in numerous other protozoans another kind of 

 propagation, budding. The most important difference is that in 

 fission the mother-animalcule divides itself completely into its 

 two descendants, while in budding it remains complete and 

 undivided, and merely separates from its body one or more 

 minute nucleated particles which gradually develop into complete 

 organisms. 



The highest perfection and the most complete conformity 

 with the phenomena of reproduction in the multicellular animals 

 is without question exhibited by the sporozoa. Their name 

 sounds strange even to-day, when science has succeeded in 

 exposing them as the most dangerous enemies of the human 

 race, for though they are microscopically minute they are capable 

 of causing the most terrible devastations in the human body. 

 Among these sporozoa are found the organisms which are the 

 dreaded cause of malaria. Their presence or absence decides 

 whether vast tracts of fertile land shall be inhabited by man or 

 not. In addition to the malarial parasite we find among the 

 sporozoa the cause of Texas fever, horse-sickness, and many 

 other diseases. Though all sporozoa do not cause serious illness 

 they all lead a parasitic life and there is hardly a class of animals 

 on which they do not prey. But as the malaria parasite is 

 undoubtedly the most important to man we will examine it 

 here in detail. 



It is more than a quarter of a century ago since Laveran, who 

 was then an army surgeon in Constantine, Algeria, discovered 

 for the first time the malaria parasite in the blood of a fever 



