the morula and ends after the hatching from the zona pellucida when the embryo makes contact with the endometrial lining of the uterus. There are only two stage 3 embryos in the Carnegie collection. There are four characteristic processes that CS3 embryos go through cavitation, collapse and expansion, hatching, and discarding of cells. Cavitation The initiation of cavitation indicates the start of CS3. This process leads to the differentiation of blastocysts into outer trophoblast cells and inner embryoblasts. Collapse and expansion This process is seen in vitro and it is not known whether this occurs in vivo. In vitro, the blastocyst rapidly collapses and slowly re-expands before hatching from the zona pellucida. Hatching During this process, the blastocyst breaks through and escapes from the zona pellucida. This process must occur prior to implantation into the endometrium. Discarding of cells TEM inspection of in vitro blastocysts has allowed us to identify two types of cells that the developing embryo apparently discards. These are sequestered cells and isolated cells. Sequestered cells are groups of cells that are located in between the zona pellucida and the trophoblast. Isolated cells are mainly found in the blastocystic cavity. Stage 4: 6 days syncytiotrophoblast cytotrophoblast amniotic ectoderm Stage 5 (a-c): 7-12 days implantation bilaminar embryonic disc primary yolk sac amniotic cavity Stage 6: c. 17 days primitive streak primitive groove chorionic villi secondary yolk sac early gastrulation Stage 7: c. 19 days gastrulation neural plate start of hematopoiesis notochord Stage 8: c. 23 days primitive pit Stage 9: c. 25 days neural groove neural folds septum transversum placode early heart Stage 10: c. 28 days pharyngeal arches #1 and #2 cardiac loop intermediate mesoderm Stage 11: c. 29 days sinus venosus mesonephric duct Stage 12: c. 30 days upper limb buds Stage 13: c. 32 days septum primum foramen primum Stage 14: c. 33 days ureteric bud Stage 15: c. 36 days Development of the Olfactory nerve and the early stage foot and hand plates Stage 16: c. 39 days lower limb buds Stage 17: c. 41 days implementation embryo in posterior uterus wall Stage 18: c. 44 days septum secundum Stage 19: c. 46 days Ectoderm: sensory placodes, lens pit, otocyst, nasal pits moved ventrally, fourth ventricle of brain Mesoderm: heart prominence, ossification continues Head: forebrain, eye, external acoustic meatus Body: straightening of trunk, heart, liver, umbilical cord Stage 20: c. 49 days Ectoderm: sensory