[The first stages of the appendix development in the human embryo]

The cecoappendicular area of 14 human embryos with a length ranging from 6 to 17 mm from vertex to coccyx (28 to 48 days old; Carnegie stages 13 to 19) was studied. The appendix is not an atrophic remain of the caecum, it develops at the same time as the caecum. The first morphological anlage of the organ is observed in human embryos from 6 to 10 mm (28 to 37 days of age; Carnegie stages 13 to 16) in the form of an elevation covered with coelomic epithelium, constituted by the mesenchyme which surrounds the anlage of the caecum. In later stages, on human embryos of 11 to 16 mm (37 to 44 days of age; Carnegie stages 16 to 18), the mesoblastic anlage of the appendix is more evident, but it is not invaded by the entoblastic cells which come from the caecum on embryos of 12 to 13 mm. On embryos of about 17 mm (48 days old; Carnegie stage 19) the entoblastic anlage of the appendix has invaded the original mesoblastic anlage. The formation of the appendix by two anlage one earlier, mesoblastic, and another later, entoblastic, is similar to that of other lymphoid organs like the sack of Fabricius in birds.