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Some anamniotic aquatic vertebrates lay eggs in a terrestrial habitat that is hostile to the survival of hatchings or larvae. These terrestrial eggs are ready and able to hatch at a particular developmental time, but do not hatch until presented with suitable conditions for aquatic larval survival. Beyond this time, hatching is possible whenever aquatic conditions occur. The duration of extended terrestrial incubation is dependent on the availability of energy for metabolism from the yolk. Extended incubation is useful for anamniotic eggs laid in terrestrial habitats where conditions suitable for larval survival arrive with unpredictable or variable timing. Examples of anamniotes with delayed hatching and extended terrestrial incubation can be found among teleost fishes, anurans, and caudate amphibians. This paper characterizes the embryonic period, compares this mode with other forms of developmental plasticity in anamniotes, evaluates the constraints and advantages of this life history mode, and examines how some fishes and amphibians are able to obtain the benefits of terrestriality for their eggs when the timing of the return to aquatic conditions is not entirely predictable.

Quantitative analyses based upon the superimposition of phylogenetic and reproductive data have revealed that viviparity has originated on at least 132 independent occasions among vertebrates, with 98 of these origins having occurred among reptiles. The viviparous lineages have given rise to at least 24 matrotrophic clades, all but four of which are anamniotes. Traditional scenarios assume progressive, gradualistic evolution from oviparity to lecithotrophic viviparity to matrotrophic viviparity. However, mammalian evidence indicates that matrotrophy can precede the evolution of viviparity. Moreover, data on reptiles seem to be consistent with a punctuated equilibrium model for viviparity and a saltatory model for incipient matrotrophy and placentation. Among the specializations for fetal nutrition, strong convergence is evident at organismal, organological, and cytological levels. Examples include yolk sac placentation, trophotaeniae, and adaptations for embryonic cannibalism. Certain lizards of the genera Mabuya and Chalcides have converged strongly on eutherian mammals with respect to morphology of the chorioallantoic placenta. Placental specializations that have evolved independently in some eutherians and matrotrophic lizards include placentomes, giant binucleate cells, deciduate maternal tissue, and chorionic areolae.

1. The organization of brain neuronal systems utilizing monoamines (primary techolamines and serotonin) as neurotransmitters and/or neuromodulators, has been studied in representatives of each major vertebrate class using fluorescence histochemical methods. This long-term investigation has led to the following conclusions: 2. (1) The monoaminergic neurons seem to have occurred early in phylogeny. These neurons, particularly the serotoninergic systems, are already well-developed in the brains of cyclostomes (lampreys) and primitive bony fishes. 3. (2) The monoaminergic cell bodies are especially abundant within the hypothalamus in anamniotes whereas they predominate at brainstem levels in amniotes. However, a similar pattern of distribution of monoaminergic cell bodies has been found in the brainstem of all vertebrates. 4. (3) The monoaminergic neurons may act as an important informative link between the cerebrospinal fluid and the neural tissue in all vertebrates through processes contacting the ventricular space. 5. (4) The monoamingeric neuronal systems are widespread and highly collateralized inall vertebrates. A single brainstem monoaminergic neuron may send divergent axon collaterals to various remote forebrain areas and, thus, could strongly influence vast neuronalassemblies. 6. (5) The terminal arborization of monoaminergic systems, particularly that of the serotoninergicneurons, displays patterns that are strikingly constant from one vertebrate class to the other. Such a constancy strongly suggests that these chemospecific neuronal systems are indeedphylogenetically ancient and subserve similar fundamental brain functions, such as the controlof sleep-waking cycle and of feeding and reproductive behaviors, in all vertebrates.