What are these structures inside a cut open Allamanda blanchetti?
The third photo gets to the heart of the matter. Here's an enlargement:

The gold-colored areas are the anthers of some of the stamens, the filaments of which are fused with the tissue of the petal.  (This is typical of the Apocynaceae, the family to which A. blanchetti belongs; they have epipetalous stamens, that is the stamens are &quot;adnate&quot; i.e. fused with the petal.) This is the male part of the flower; the anthers have the pollen grains which the plant is trying to get transferred to another flower.
Above the knife is the style.  This part of the female structure is a hollow tube which runs from the openings in the stigma (the out of focus green blob) down to the ovary (not in photo). The central part of the fourth photo is presumably the ovary (missing its style). The ovary contain the ovules which will become seeds after fertilization from pollen which sticks to the stigma and grows a tube down the style.