William Bateson was an obsessive observer of animal oddities, and at some point in his herculean survey of museum collections leading up to his monumental 1894 monograph (Materials for the study of variation), he noticed a peculiar trend among the preserved specimens (mainly insects) that possessed extra legs: multiple legs that branched from the same socket tended to be mirror images of their adjacent neighbors. He did not know why. These symmetry relationships have come to be known as Bateson's rule, and they have defied a satisfactory explanation for 125 years. In the past few decades, tantalizing clues have emerged from various lines of investigation, and those lines have converged on a possible solution. An attempt is made here to fit all of those clues together to form a coherent picture of the etiology. Two case studies have proven to be pivotal: a fly mutant whose extra legs are caused by patches of dying cells and a frog syndrome whose extra legs are caused by a parasitic flatworm. The conclusion reached is that the extra legs of insects and vertebrates obey Bateson's rule for the same reason, but that reason has nothing to do with the specific molecules in their signaling pathways. Rather, it is an emergent property of the circuitry of the pathways and their polarized alignments along the limb axes. A parade of theoretical models have tried and failed to crack this mystery in the past, and they are reviewed here as part of the narrative.

中文翻译：

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关于贝特森法则的思考：解决一个古老的谜题，即为什么多余的腿是镜像对称的。

威廉·贝特森（William Bateson）是痴迷于动物奇观的观察者，在对1894年巨著（研究变异的材料）导致的博物馆收藏进行严密调查的某个时刻，他注意到保存下来的标本（主要是昆虫）中的奇特趋势。具有额外的支腿：从同一插座分支出的多支支腿往往是其相邻邻居的镜像。他不知道为什么。这些对称关系已被称为贝特森法则，并且在125年的时间里一直没有令人满意的解释。在过去的几十年中，各种各样的研究线索都产生了诱人的线索，这些线索已经汇聚成一个可能的解决方案。这里尝试将所有这些线索融合在一起，以形成病因学的连贯图景。两个案例研究被证明是至关重要的：一个蝇突变体，其多余的腿是由垂死的细胞斑引起的；还有青蛙综合症，其多余的腿是由寄生的扁虫引起的。得出的结论是，昆虫和脊椎动物的多余腿出于相同的原因而遵守贝特森法则，但该原因与它们的信号传导途径中的特定分子无关。相反，它是通路电路及其沿肢体轴的极化排列的新兴属性。过去，许多理论模型都曾尝试并未能破解这一谜团，在此将它们作为叙述的一部分进行回顾。得出的结论是，昆虫和脊椎动物的多余腿出于相同的原因而遵守贝特森法则，但该原因与它们的信号传导途径中的特定分子无关。相反，它是通路电路及其沿肢体轴的极化排列的新兴属性。过去曾有许多理论模型尝试并未能破解这个谜团，在此将它们作为叙述的一部分进行回顾。得出的结论是，昆虫和脊椎动物的多余腿出于相同的原因而服从贝特森法则，但该原因与它们的信号传导途径中的特定分子无关。而是，这是通路的电路及其沿肢体轴的极化排列的新兴特性。过去曾有许多理论模型尝试并未能破解这个谜团，在此将它们作为叙述的一部分进行回顾。