BEPRODUOTION AND EMBRYOLOGY. 13 



Reproduction. — The simplest form of reproduction is 

 cell-division, one cell budding or separating from another. 

 This mode of growth is called self-division or fission. 

 Where one cell separates from another, the separating part 

 being smaller than the original cell, or where a number of 

 cells separate or bud out from a many-celled animal, such as 

 a Hydra, the process is called gemmation. A third mode of 

 reproduction is sexual, the sperm-cell of the male coalescing 

 with the nucleus of the egg ; the commingling of the pro- 

 toplasm of the two nuclei resulting in a series of events 

 leading to the formation of a germ or embryo. 



Embryology is, strictly speaking, a study of the develop- 

 ment of animals from the beginning of life of the egg up to 

 the time the animal leaves the egg or the body of the parent 

 • — namely, up to the time when it begins to shift for itself ; 

 but the term embryology may also be applied to the grow- 

 ing animal from the egg to the adult condition. Many of 

 the lower animals undergo a metamorphosis, suddenly as- 

 suming changes in form, accompanied by changes in habits 

 and surroundings ; so that at different times it is, so to 

 speak, a different animal. For example, the caterpillar 

 lives on solid food, crawls on the ground, and has a worm- 

 like form ; it changes to a chrysalis or pupa, lying quies- 

 cent, taking no food ; then it changes to a butterfly and 

 flies in the air, either taking no food or sipping the nectar 

 of flowers : in all these three stages it is virtually different 

 animals with different surroundings. Many animals besides 

 insects have a metamorphosis, and their young are called 

 larvae ; thus there are larval polyps, larval star-fish, larval 

 worms — these larvae often differing remarkably in form, 

 habits, and in their environment or surroundings, as com- 

 pared with the mature or adult forms. 



Classification. — After thoroughly studying a single ani- 

 mal, its external form, how it acts when alive, its external 

 and internal anatomy after death, and the development of 

 other individuals of its own species, the student is then ready 

 to study the classification of animals. 



The best method of studying classification, or Systematic 

 Zoology, is to make an exhaustive examination of one an- 



