# Field Homology in the Brain of Vertebrates

## TL;DR

The paper explains how field homology helps compare brain structures across vertebrates by focusing on embryonic origins rather than just form or function.

## Contribution

It introduces field homology as a framework for understanding brain homology across species, emphasizing embryonic developmental fields over morphological similarity.

## Key findings

- Field homology allows tracing homologies across genomic, embryonic, and adult levels despite variant final derivatives.
- Similarity of form or function is not necessary for homology and can be misleading due to homoplasy.
- Embryonic developmental fields have intrinsic cohesivity and communication mechanisms that lead to specific morphostatic fates.

## Abstract

The concept of field homology is an important notion in comparative anatomy and neuroscience (of interest to applied and clinical neuroscience in judging the validity of animal models of the human body and brain parts). It smooths over the morphologic hypotheses of homology referring to either embryonic or adult animal forms (the latter refer to the fundamental sameness of body or brain parts according to a common Bauplan and shared developmental/molecular mechanisms, usually comparable between vertebrate animal models and the human). It applies wherever manifestly homologous embryonic primordia produce in two species variant (non-similar) final derivatives. It also facilitates tracing homologies between different levels of complexity (genomic, embryonic, and adult). We emphasize that similarity of form or function is not required (and may be misleading in cases of homoplasy) in a hypothesis of homology either on embryonic or adult forms. Homologs usually have some variant aspects. Fundamental sameness results from the embryonic existence of regulative developmental fields, able to achieve a specific morphostatic fate via a variety of mechanistic routes, thanks to their intrinsic cohesivity, coherence, and diverse intercellular communication mechanisms. We conclude that field homology is a valuable heuristic recourse, particularly for the comparative evolutionary analysis of the complex brain structure, for which there is little fossil evidence.

In this commentary, we discuss the concept of field homology, particularly as applied to the brain in comparative neuromorphology. We emphasize its roots in the topological properties of the Bauplan notion (organization plan) as well as in the still scarcely understood concept of developmental fields, and we criticize their modernly frequent erred substitution by the search of similarity of characters. We defend the logical causal connection of embryonic homology with adult homology, irrespective of the regulatory aspects of ontogeny.