细胞恒定（Eutely），或称细胞恒定，是指在动物的成年形态中，有特定数量的细胞，在不同的物种中是一致的。 只有非常小的动物，通常尺寸小于1毫米，总细胞数小于10万个，才是恒定的。 显示恒定的动物的一些例子包括轮虫、线虫、缓步动物、胃肠动物和双壳动物。 虽然所有恒定的生物体都有细胞恒定的共同特点，但其细胞大小可能有许多数量级的差异，1000细胞的生物体可以从0.1mm到超过10mm。

Eutely occurs when the initial embryonic cells each come with a limit for total divisions. After this limit is reached, the organism can only become larger due to cell enlargement rather than cell division. Eutely can be contrasted with most other animals which have an undefined number of total cells. Eutely may also occur at the organ level, such as in the eutelic nematode Ascaris, which always has 162 neurons. Eutely can be helpful for scientists seeking to analyze the specific developmental history of each specific cell in an entire organism, or see how an organism's genome (genotype) relates to its biological form (phenotype). It makes a given organism easier to understand, the most notable example being in the nematode C. elegans, one of the most-used and best-understood model organisms in science, alongside mice and fruit flies.

最小的优生生物是简单的蠕虫，如dicyemids，寄生在头足类动物的肾脏附属物中，长度从0.1-9.0毫米不等。 一些藻类群落，如Eudorina，单细胞和多细胞生物之间的进化中介，显示出优生性，群落大小总是32、64或128个细胞。 这些细胞在一个称为法定人数感应的过程中相互进行化学交流，确保群落中细胞总数随时间保持不变。