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

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

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

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