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Much more is known of the role of immediate proximate factors in amphibian breeding, though our knowledge is derived from field observations since there has been little experimentation. The breeding behavior of frogs in more xeric areas follows falling rain and its accumulation in ponds and pools, whereas species living in mesic environments respond to temperature. The temperature of the pools and of the air is important, as many mesic species will only call within specific and narrow ranges of water tempera­ tures.

This struc­ ture, unlike that of higher vertebrates, offers little nutriment to the egg. In all amphibians the ovary is gymnovarian and the eggs pass through the body cavity before entering the ostium ( opening to the oviduct). The eggs then pass through the enlarged oviduct which places various jelly layers around them. They may be stored there for varying periods but eventually pass to the exterior at oviposition. In most salamanders there are cloacal glands at the end of the oviduct which are modified to form a spermatheca, a special pocket which receives the spermatophore.

At the same time the two layers of cells surrounding the follicle, the theca interna and theca ex­ terna expand and may differentiate into various sublayers. De­ pending on the species of reptile, the thecal layer may or may not grow inwardly fingerlike projections into the corpus luteum with fibroblastic cells and blood vessels. The corpus luteum of reptiles lasts varying lengths of time depending on the degree of viviparity. External Indications of Sex Although strictly not external except during copulation, the intromittent organs of male reptiles are the major indicators of sex.

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