115 GENERAL CONCEPTS 



celled animals and plants; the cell division involved is mitotic. Coe- 

 lenterates typically reproduce by budding; a small part of the parent's 

 body becomes differentiated and separate from the rest. It develops 

 into a complete new individual and may take up independent existence, 

 or the buds from a single parent may remain attached as a colony of 

 many individuals. 



Salamanders, lizards, starfish and crabs can grow a new tail, leg or 

 other organ if the original one is lost. When this ability to regenerate 

 the whole from a part is extremely marked it becomes a method of re- 

 production. The body of the parent may break into several pieces and 

 each piece then develops into a whole animal by regenerating the miss- 

 ing parts. A whole starfish can be regenerated from a single arm. 



One class of protozoa, the Sporozoa, characteristically reproduce 

 asexually by means of spores, special cells with resistant coverings which 

 withstand unfavorable environmental conditions. An interesting ex- 

 ample of reproduction by spore formation is the parasitic protozoan, 

 Plasmodium, which causes malaria. The organism has a complex life 

 cycle involving man and the Anopheles mosquito (Fig. 6.1). The malaria 

 organism enters the human blood stream when the mosquito bites the 

 man, and attacks and enters the red blood cells. Within the red cell 

 each Plasmodium divides into 12 to 24 spores, each of which is released 

 when the red cell bursts later on. The released spores infect new red 

 cells and the process is repeated. The simultaneous bursting of billions 

 of red cells causes the malarial chill, followed by fever as the toxic 

 substances released penetrate to other organs of the body. If a second, 

 uninfected mosquito bites the man, it will suck up some Plasmodiwn 

 spores along with its drink of blood. A complicated process of sexual 

 reproduction ensues within the mosquito's stomach and new spores are 

 formed, some of which migrate into the mosquito's salivary glands and 

 are ready to infect the next man bitten. 



34. Sexual Reproduction 



Sexual reproduction is characterized by the development of a new in- 

 dividual from a zygote, or fertilized egg, produced in turn by the fusion 

 of two sex cells, an egg and a sperm. Certain protozoa have a compli- 

 cated process of sexual reproduction in which two individuals come to- 

 gether and fuse temporarily along their oral surfaces. The nucleus of 

 each one divides several times before one of the resulting daughter 

 nuclei migrates across to the other animal and fuses with one of its 

 nuclei. Following this the two animals separate and each reproduces 

 asexually by fission. Paramecia are not differentiated morphologically 

 into sexes, but T. M. Sonneborn has shown that there are distinct, 

 genetically determined mating types. A member of one mating group 

 will mate only with some member of another group. 



Meiosis. The mitotic process of cell division is remarkably con- 

 stant and ensures that the number of chromosomes per cell will remain 

 unchanged through successive cell generations. The fusion of an egg and 

 a sperm to form a fertilized egg would result in a doubling of the 



