22 GERM-CELL CYCLE IN ANIMALS 



spermatozoa of the fowl are placed in a hanging drop 

 of white of egg or in yolk they undergo a transfor- 

 mation into nuclei. The possibility that a sperma- 

 tozoon may give rise to an embryo without the help 

 of an egg is recognized, but this has not yet been 

 accomplished. 



THE LIFE CYCLES OF ANIMALS. The life cycle 

 of an animal has considerable influence upon the 

 course of the germ-cell cycle. In all animals that 

 are produced by the sexual method the beginning 

 stage in the life cycle is a mature egg, either fertilized 

 or unfertilized according to the species. Animals 

 which develop asexually, on the other hand, begin 

 their cycle with the first recognizable evidence of 

 budding or fission. As a rule budding or fission are 

 sooner or later interrupted by the formation of sex 

 cells, hence the life cycle of such animals may be 

 considered to extend from the mature egg to that 

 stage in the life history of the species when mature 

 eggs are again produced. Such a life cycle consists 

 really of two or more simple life cycles represented 

 by individuals differing from one another in both 

 structure and method of reproduction. As examples 

 of some of the principal types of life cycles we may 

 select certain insects and ccelenterates. 



A very simple life cycle is that of the wingless 

 insects of the order APTERA. The young, when they 

 hatch from the egg, are similar in form, structure, 

 and habits to the fully grown individual and undergo 

 no perceptible changes, except increase in size, 

 until they become sexually mature adults. In 



