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).

The fertilized egg cell divides, first vertically, then horizontally; it divides again and again and again, until one recognizes that there is now a tiny berry-like cluster of cells, small black ones uppermost, slightly larger, creamy white ones below. There is obviously a polarity in the egg with a northern pole, for what is termed the “animal hemisphere” and a southern pole for the “vegetal hemisphere”. Then the exciting process of “invagination” takes place and it looks as if a tennis-ball is pushed inward at one spot. Around the dorsal edge from where the inward migration of the cells occurs (the so-called “blastoporus”) the coordination centre is located: here it is “decided” which group of cells is to become limb, liver, nervous system, kidney or tail, etc. Not long after the invagination, a developmental stage known as the neurula becomes apparent as a groove along the length of the dorsal side of the embryo with infolding edges that turn into the neural tube, i.e., central nervous system of the animal. All of this can be so beautifully observed in the amphibian egg. Soon after the neurula’s completion a head with tiny eyes, gills and the tail end can be made out.

The fate of the blastopore itself in the future development of the embryo has led to the acceptance by zoologists of two major lines of animals: those in which the blastoporus later becomes the mouth of the organisms (as in the “protostomes”) and those in which the mouth is later formed at the other end of the body and the blastoporus turns into the anus (as in the “deuterostomes”). Based on this scenario, we humans and other vertebrates including our frogs, toads and newts are deuterostomes and, thus, actually more closely related to starfish, sea urchins and sea squirts than to the insects, crayfish, cuttlefish and jellyfish, the latter four all being representatives of the protostomes.

There are further reasons like the origin of the body cavity (the so-called coelom) and the formation of the muscles which support this view of the basis dichotomy of all multicellular animals into two camps. But back to our observations on the development of our frog, toad, and newt eggs: all this zoology is quite irrelevant, isn’t it, for the simple admiration of how from an undifferentiated, seemingly lifeless blob of jelly at the start an active, tail-flicking tadpole emerges at the end a couple of a weeks or a few days later.

© Dr V.B. Meyer-Rochow and, 2018.
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