This section contains mainly endometrium in the secretory phase but you may notice a cyst-like structure.  This is an egg – that is, an implanted fertilized egg that at this stage consists of an inner cell mass (not present in the section) and an outer cell mass forming the placenta.

Secondary villi, as seen at early stage here, appear 15-21 days after the fertilization. Since the fertilized egg takes about 6 days to reach the uterine cavity, this one has presumably developed for 1-2 weeks after implantation.