1. Internal Development
Placental internal development is the support of the development of an embryo inside the body of the parent and results in the birth of live offspring
Placental mammals (including humans), some sharks, snakes, cockroaches, and scorpions
Thought to be the most highly developed form of offspring development

2. Internal Development
The development of the young is supported either parent in or on any part of their body
Other shark species exhibit this form of internal development and rely on a yolk sack rather than the placenta
In some frogs (young develop in mouth or stomach) and in seahorses (young develop in marsupium), the zygotes are retained for protection during development but there is no interaction between the zygotes and the parent

3. Environmental Sex Determination
Environmental sex determination is the establishment of sex by non- genetic factors (genotypic sex determination)
Temperature-dependent sex determination is experienced by some reptiles in which sex is determined by thermal conditions during embryonic development
The sex of Crocodiles and alligators is exclusively determined by temperature
Scaled reptiles (snakes and lizards) and turtles exhibit a combination of environmental and genotypic sex determination

4. Environmental Sex Determination
The thermo-sensitive period is the stage during development when sex is determined
Pattern 1: single transition zone, occurs in most turtles
males produced below the temperature threshold
Females produced above the temperature threshold
Pattern 2: two transition zones, occurs in lizards and crocodilians
Males produced in the middle temperatures
Females produced at the extremes