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- 
tozoén may give rise to an embryo without the help 
of an egg is recognized, but this has not yet been 
accomplished. 
Tur Lire Cycues or 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 
