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Ovoviviparous animals also brood their embryos internally, but in this case there is
no placental connection, and the unborn embryo is nourished by yolk within the egg.
This is the situation in some fish and amphibians. Ovulation, as defined by the release
of the mature egg from the ovaries, does not follow oocyte maturation in these ani-
mals: fertilization and embryo development take place in the ovarian follicles. The
embryos then hatch internally and are born as live young (Jalabert 2005 ).
It is generally the case that the fertilizing sperm stimulates development of an egg
into an embryo at the time of fertilization. Interestingly, exceptions exist. Eggs of
different types of teleost fishes use different triggers for activation (Iwamatsu 2000 ).
In internally fertilizing fishes, the sperm enters the egg and induces activation in the
ovarian follicle. Some fishes with external fertilization conserved the ability to start
development upon sperm penetration. Eggs of medaka (Japanese killifish) and stick-
leback, for example, activate in response to sperm penetration (Oryzias-type eggs).
However, eggs of the seawater teleost fish Alcichthys can undergo sperm penetration
and activation by sperm only after exposure to seawater, following spawning
(Alcichthys type). The most extreme situation can be observed in salmon- and
Carassius-type eggs, where the function of activating the egg has completely shifted
from the sperm to the tonicity of the aqueous medium. These eggs can undergo acti-
vation in the absence of sperm, simply by being immersed in hypotonic saline. The
identity of the trigger that stimulates activation in the absence of sperm is a matter of
debate; mechanical stress, osmotic shock, and changes in the ionic environment or
pH have all been suggested as potential causes. Mechanical stress caused by egg
laying induces activation in the wasp Pimpla turionellae (Went and Krause 1974 );
stretch-activated channels are expressed in Xenopus eggs (Yang and Sachs 1989 );
and the presence of stretch-activated K+ channels has been described in the fish
Misgurnus fossilis (Medina and Bregestovski 1988 ). Additionally, osmotic shock
may occur when during spawning, the eggs are released from the protein-rich ovar-
ian fluid into the freshwater environment. However, the role of these stimuli, if any,
during fish egg activation is yet to be demonstrated (Webb and Miller 2013 ).
Following sperm deposition, the spermatozoa first reach the outer layers of the
egg. Fish eggs have a thick and mechanically tough shell, the chorion (vitelline
envelope; Iwamatsu 1969 ). They possess a specialized opening, the micropyle on
the chorion above the animal pole; the fertilizing sperm must swim through this
hole to gain access to the egg surface (Hart and Donovan 1983 ). The micropyle also
limits the number of sperm reaching the plasma membrane. In amphibians, the gen-
eral pattern of fertilization is different between anurans and urodeles. Most anuran
eggs are inseminated externally after oviposition (Elinson 1975 ), whereas urodele
eggs are generally inseminated in the female’s cloaca. Such females receive a sper-
matophore, a ball of sperm deposited by the male. Females store it in the sperma-
theca near the cloaca, and the sperm released from the spermatheca fertilizes the
egg just before oviposition (Sever and Brizzi 1998 ). Amphibian eggs are surrounded
by a vitelline envelope and 3–6 layers of jelly coat; the sperm must penetrate these
investments in order to reach the plasma membrane. Sperm can only penetrate the
animal half of the anuran egg but enter both the animal and vegetal halves in uro-
deles. Finally, only a single sperm fertilizes the egg of most anurans, while most
Z. Machaty et al.