This photo shows eggs that have been recovered from feces of an infected rat, then placed on a glass slide and ruptured with cover-glass pressure. This simulates the natural ingestion of the eggs by a beetle first intermediate host, where the mouthparts would rupture the outer egg membranes.
The eggs in this photo are in three different states. Several eggs, such as the two in the lower right corner, are intact and appear as they would within the gravid proglottid shed by the adult. The oncosphere is the small circular body at the centre, surrounded by several thick outer egg membranes which provide protection from the external environment.
At the top of the photo are two eggs in the process of rupturing. In the centre left of the photo are three oncospheres that have emerged from the tough outer egg membranes. They are still surrounded by embryophore membranes, which have expanded slightly since their release from the egg. These membranes are difficult to rupture by mechanical means, but they can be broken down by enzymes within the beetle gut, liberating the small oncosphere within them. In several of the oncospheres, including some that are still in the egg, you can see pairs of thin, dark rods. These are embryonic hooks which help the oncosphere penetrate through the gut wall of the intermediate host, and are present only in cestodes.