Figure 8.

Schematic comparison of the different forces during ontogeny that lead to the different hypothalamic anatomy. In this hypothetic scheme the situation between mammals and non-mammals (mainly based on our results in the development of the amphibian hypothalamus) are represented. Three main forces are supposed to act in a sequential manner and differently in each vertebrate group. The first force (1) to act is the flexure of the neural tube (A). In mammals, the longitunal axis bends almost 90° forming a sharp flexure and the rostral tube is moved to a “ventral” position (B), whereas in non-mammals this angle seems to be less pronounced (C). Then, a second morphological force acts over this longitudinal axis that is already partially bent, which is produced by the telencephalic evagination (2). In the case of mammals this second force acts equally on the caudal (hp1) and rostral (hp2) hypothalamic domains, so its main effect would be the flattening of the hypothalamic territory. However, in the case of non-mammals, the strength caused by the telencephalic evagination would be mainly pushing the rostral (hp2) hypothalamic domain, which helps to turn more “ventrally” the hypothalamus. Finally, a third force is the hypothalamic evagination (3). In mammals this third strength is contributing to the elongation of the hypothalamic territory and, in the case of non-mammals this force is also contributing to pronounced hypothalamic modification.