CHAPTER XLIII 



MULTIPLICATION IN ANIMALS 



Questions. 1. How do different kinds of animals multiply? 2. Do 

 any species of animals multiply in more than one way ? 3. Does every 

 animal start as a single cell ? 4. Do all animals take care of their young? 

 5. Are any animals able to take care of themselves the moment they are 

 born ? 



351. Cell division. One-celled animals (protozoa), like one- 

 celled plants, multiply by simple cell division ; and many-celled 

 animals, like many-celled plants, grow in size (i) by the in- 

 crease in the number of cells through cell division, and (2) by 

 the enlargement of the cells themselves through the assimilation 

 of food. Again, as in plants, cell division in animals results in 

 the heahng of wounds and breaks, as when your hand is cut or 

 when a bone is broken. This healing of skin or bone and other 

 tissues is quite common among all kinds of plants and animals, 

 and may be considered as a growth in response to stimulation 

 set up by injury. 



Not all kinds of tissues will produce new cells of the same kind. An 

 injury to the brain will usually heal up by the formation of a scar of 

 connective tissue. A wound on a tree often results in the formation of 

 calhis instead of growing wood and bark (cambium). 



352. Regeneration. We saw that a portion of a plant is capa- 

 ble, under certain conditions, of regrowing the parts lacking to 

 make a complete plant. A somewhat similar process is to be 

 observed among certain animals. If the fore part of an earth- 

 worm is cut off, a new fore part is formed. If the hind end is 

 removed, a new tail may be grown (Fig. 200). This process of 

 regrowing a lost part is called regeneration, and is in some ways 

 similar to the healing of a wound, since it depends on cell divi- 

 sion ; but it goes much farther. 



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