We usually think "mammal" when we hear the word “animal.” In fact, we and this Rhinoceros beetle are both animals.
In the Animal Kingdom, a small swimming sperm makes it to a large egg, spurred on by its undulating tail. The fertilized egg repeatedly divides to form, in the initial stage of embryo development, a hollow sphere of cells — the animal blastula. This blastula is the defining trait of animal-hood.
The bodies of animals are individualized with special cell-to-cell connections. As the embryo cells divide, some must form alliances, while most others die on a preprogrammed cue. If these cells do not commit cell-icide in the proper fashion, no animal body develops.
University of Montana--Missoula. Environmental Studies Program
© 1997 Stiftung Drittes Millennium
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Liebes, Sid; Mittelstadt, Laurie; Waugh, Barbara; and Brynes, Lois, "Panel 49: What Are Animals?" (1997). A Walk Through Time - From Stardust To Us. 49.