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Frog Embryology

The Egg

The frog egg is a huge cell; its volume is over 1.6 million times larger than a normal frog cell. During embryonic development, the egg will be converted into a tadpole containing millions of cells but containing the same amount of organic matter.


Entrance of the sperm initiates a sequence of events:


The zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned off, by cytokinesis, in separate, and ever-smaller, cells. The first cleavage occurs shortly after the zygote nucleus forms.

The second cleavage forms the 4-cell stage. The cleavage furrow again runs through the poles but at right angles to the first furrow.

The furrow in the third cleavage runs horizontally but in a plane closer to the animal than to the vegetal pole. It produces the 8-cell stage.

The next few cleavages also proceed in synchrony, producing a 16-cell and then a 32-cell embryo.

However, as cleavage continues, the cells in the animal pole begin dividing more rapidly than those in the vegetal pole and thus become smaller and more numerous.

By the next day, continued cleavage has produced a hollow ball of thousands of cells called the blastula. A fluid-filled cavity, the blastocoel, forms within it.

During this entire process


The start of gastrulation is marked by the pushing inward ("invagination") of cells in the region of the embryo once occupied by the middle of the gray crescent. This produces: As gastrulation continues, three distinct "germ layers" are formed: Each of these will have special roles to play in building the complete animal. Some are listed in the table.
Germ-layer origin of various body tissues
skinnotochordinner lining of gut, liver, pancreas
brainmusclesinner lining of lungs
spinal cordbloodinner lining of bladder
all other neuronsbonethyroid and parathyroid glands
sense receptorssex organsthymus

The Spemann organizer (mostly mesoderm) will:

See Organizing the Embryo: The Central Nervous System to read more about the Spemann organizer and the chemical signals responsible for converting the ectoderm above the notochord into neural folds and, in due course, the brain and spinal cord.


Although the various layers of cells in the frog gastrula have definite and different fates in store for them, these are not readily apparent in their structure. Only by probing for different patterns of gene expression (e.g., looking for tissue-specific proteins) can their differences be detected.

In due course, however, the cells of the embryo take on the specialized structures and functions that they have in the tadpole, forming neurons, blood cells, muscle cells, epithelial cells, etc., etc.


At the time the tadpole hatches, it is a fully-formed organism. However, it has no more organic matter in it than the original frog egg had. Once able to feed, however, the tadpole can grow. It gains additional molecules with which it can increase the number of cells that make up its various tissues.
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17 May 2011