biology zoology blog frog egg embryon (1)

A Frog’s Egg

What can a frog’s egg teach us

Hardly anything in zoology could be more exciting than to observe how from an egg cell a whole new individual develops. Unfortunately for the curious person very often the developing egg is hidden from view as in mammals and sometimes it is so small that it is impossible to examine what precisely goes on. But there are some animals which allow even children without the aid of a microscope to observe the embryo and how it grows inside the egg. One example are freshwater pulmonate snails like Planorbarius corneus (the ramshorn snail) or Lymnaea stagnalis (the common pond snail). Their eggs, attached in clusters of up to 40 or so on the glass walls of an aquarium make observations easy. But other, and even bigger and therefore more suitable eggs allowing one to follow the changes that go on inside them are those of frogs, toads and newts. In the gelatinous eggs of these amphibians, rice grain sized in newts but up to the size of peas in frogs, one can see the entire developmental process through the transparent egg membranes virtually with the naked eye (although a hand lens would help, of course).

penguin easter egg meyer rochow nutrition

Easter post : The Avian Egg : Brittle, Delicate, yet Firm

The Egg : Nutritious and perfectly shaped

We are easily fascinated by the largest, fastest, strongest, ugliest…., well superlatives generally and when you examine animal tissues, you can, of course, classify cells according to their size, shape, volume, etc. The longest cells in the human body are, no doubt, certain nerve cells with their projections called axons. In the giraffe or in a whale such neurons may easily be several metres long. But the most voluminous cells overall is nowadays, after the demise of egg-laying dinosaurs, the bird egg. And while we are at it: the smallest eggs, only 1 cm in length and 0.37 g in weight, are laid by the Jamaican hummingbird Mellisuga minima.

An unfertilized egg of a bird is a single cell (tasty and nutritious), with one nucleus and a massive amount of yolk. The semi-liquid egg content is contained in a wonderwork of porous lime on a matrix of interwoven organic fibres, which together (lime and organic fibres) make up the egg shell. The shape of the egg is marvellously practical – not only from the point of laying it, but as a compromise to strength and durability. Egg shells are often cryptically coloured and therefore less conspicuous; they also have to be sufficiently strong to resist predators. But the shell’s most important task is to prevent the semi-liquid content from oozing out through the tiny pores in the shell, but at the same time allowing gas exchange of the breathing embryo inside the egg to take place without leading to water loss, i.e., dehydration. It was shown that pigeon eggs laid under very dry conditions, exhibited 30-40% less water evaporation pressures (correlated with the total number of gas exchange pores and a greater shell thickness) than eggs, which were laid under conditions in which the relative humidity of the air was not elevated and I have found that penguins also lay eggs with relatively thick egg shells, perhaps in response to the dry Antarctic climate.

On the blunt end of each egg is an air chamber whose physiological role in respiration and internal humidity control only recently has been worked out. Although present day ostriches lay by far the biggest eggs, extinct New Zealand moas had larger eggs still and the “elephant bird” Aepyornis of Madagascar, which died out only about 350 years ago, even laid eggs that weighed 10 kg, were 34 cm long and had a volume of 160 chicken eggs. There is, however, an upper size limit for an egg, because a greater volume requires thicker and stronger shells and one arrives at a point, where a hatching chick simply would not have the power to escape from its cradle – unless the parent bird helped by breaking the egg from the outside.

Eggs are a powerful symbol of life, growth, love and fertility and they play a variety of cultural roles: famously decorated Fabergé eggs come to mind; eggs as wedding gifts (the Romans knew: “omne vivum ex ovo”), but also as a food item to be avoided (by Brahmin Hindus, for example). Northern Australian Aborigines, however, are known to have feasted for weeks almost entirely on eggs during the breeding season of the magpie goose and at Easter finding, collecting and eating eggs, maybe not for weeks, unless they are made out of chocolate, is still lots of fun – that is if you are not Chinese and prefer egg jam or eggs buried for a few months in the soil with clay, ash and salt until they smell so beautifully mature and shine so deliciously enticing in sullen brown, green and purplish hues when cut in half!

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Is Ayu waiting for the thick shell penguin egg?

© Dr V.B. Meyer-Rochow and, 2016.
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