Ontogeny as a whole: Some developmental trends

Abstract

In many animal groups, rudimentary organs of adult organisms appear at early stages of egg cleavage, when there is no trace of morphological differentiation. More than half a century ago, I. I. Schmalhausen developed a profound concept of integrity of the organism in the course of development. He emphasized that mutual adaptation of organs is based on correlations, involving the so-called nonhereditary modifications, which are transformed into hereditary traits by means of natural selection. There are many exceptions to the biogenetic law, as it was understood by de Beer. In particular, embryo of the pouched tree frog Gastrotheca shows a retarded development and is spread over the yolk surface. As a result, it becomes similar somewhat to embryos of amniotes. After the metamorphosis, Xenopus returns to ammonotelism. The integrity of ontogeny at the molecular level has much in common with the neutrality theory.