A Review of Placentation among Reptiles with, particular regard to the Function and Evolution of the Placenta.
Authors: Weekes H. Claire 
Published: August 2010

Summary.

This is a reveiw of reptilian placentation and other phenomena in the reptilian reproductive cycle concerned with viviparity.

There are three distinct types of placentation so far decribed for reptiles:‐

Type (i). A simple where the partial degneration of maternal and embryonic epithelium allows for a close apposition of maternal and embryonic blood streams. The lizards with this type of placents have no apparent reduction in the yolk‐content of thier eggs at the time of ovulation. These Lizards are Lygosoma (Hinulia) quoyi, L. (Hemiergis) quadridigitatum, Tiliqua scincoides, t. nigrolutea, and from Giacomini's decription Chalcides ocellatus, as well as the snakes Denisonia superba and D.suta.

Type (ii.). A simple type where the maternal capillaries are raised into small folds. Ther grooves between the folds are lined with glandular epithelium, but the cdapillaries themselves are exporsed at the surface of the folods. The underlying chorionic ectoderm may be thickened and glandular. No early stages were available for and examination of the yolk‐content of the eggs. The lizards with this type of placenta are Lygosoma (Liolepisma) pretiosum, L. (L.) ocellatum, and L. (L.) metallicum.

Type (iii.). The most specialized placenta so far described for reptiles, where the uterine wall, over a distinct elliptical area beneth the main longitudinal utering blood‐vessels, is raised into a series of folds filled with capillaries and lined with thickened glandular epithelium and is underlain by an elliptical embryonic area of much thickened chorionie ectoderm. The Lizards with this type of placements have a reduced yolk‐content at the time of ovualion. Thes lizards are Chalcides tridactylus, Lygosoma (Liolepisma) entrecasteauxi, and L. (L.) weekesæ.

It is suggested that (α) in reptiles with and obvious reduction in the yolk‐content of their eggs at the time of ovulation, and its invaribale accompaniment by a speciallized folded glandular area of allanto‐placentation, the function of this specialized placenta, is nutrition; that (b) the funciton of the yolk‐sao placenta is possibly to supply water to the developing egg: and that (c) the function of the simple placenta, so consistently present in lizards with eggs with an apparently yolk‐content, is respiration.

The condition of the large yolk‐laden egg held tighly in a stretched utersu is obviously the most primitive expression of viviparity among repitles. The condition of a samll egge with a reduced yolk‐content is more advanced, and is obviously but a step behind the yolkless egg, such as occures among the eutherian mammals. Hence it is suggested that the function of fixation, although one of the first concerns of the trophoblast of the small mammalian egg to‐day, has probably originated secondariyly to some other funciton, presumably respiration.

It is suggested that with a decrease in the yolk‐content of the egg the yolk‐sac placenta would gradually lose significance as a water absorbing organ, but the virture of its extra‐embryonic circulation, a placenta to funciton for respirtion or nutrition, or both, could be evolved, and it is conceivable that the mammalian yolk‐sac placenta could have been evolved along lines similar to theses.

The specialized placenta. type (iii.) is considered to have evolved from the simple conditon described as type (ii.), and it is thought that the placenta in ungulates (the mammalian non‐deciducte plancenta) may have had origin form some such simple type as type (i.), through a stage in placentation similar to type (iii.), which Giacomini likened to a cow's cotyledon in early stages of development.

The facts are condsidered to justify the conclusion that among reptiles placentation has arisen independently development of similar types of placentas is common.

It ws found that the proportion of viviparous to oviparous reptiles on the Great Dividing Range and the inland plain of south‐easten Australia was extradinarily high, and that the number of oviparous species only approached the number of viviparsous speices on the lower slopes of the Dividing Range, and on the coastal plain.

In the Pyrenees and the French Alps by far the majority of lizards found at high altitudes were viviparous. the oviparous species were practically confined to the mountaind slopes up to about 3000‐4000 feet above sea‐level.

It is suggested that the failure of oviparous species to establish themselves at 4000 feet and over above sea‐level amy be caused by cold interfering with development of the eggs in the nest.

All viviparous speices found on, or recorded from, the inland plain, the mountain slopes, ro the coastal plain of south‐eastern Australia also occur at least 4000 feet above sea‐level on the Great Dividing Range.

Two viviparous speices with teh most highly specialized allanto‐placeta and reproductive cycle yet recorded for reptiles in Australia, and only equlled by the lizard C. tridactylus in Italy, are restricted to 4000‐7000 feet above sea‐level.

Therefore, it is suggested that in south‐eastern Australia, at least, the factros determining the adoption of viviparity may be either (α) definitely associated with the altitudes to which these lizards are restricted or (b) may work most efficently under the conditons existing at such altitudes.

Cold is suggested as the most likely external factor associated with high altitudes that may influence either directly or indircectly the adoption of viviparity.