The alimentary system has an expanded stomach, pancreatic primordia and a gall bladder. Soon after laying, five primary brain vesicles are present, the trigeminal ganglia CN5 as well as CN7, CN8, CN9, CN10, CN11 and CN12 are well developed. The embryo at this stage is in a period of rapid modelling of the major early organ primordia of the nervous system, cardiovascular system, excretory system, and somite-derived components of the body wall. The full-term, subspheroidal, intrauterine egg of the platypus has a major axis of about 17 mm and contains a flat, 19-20 somite, neurula-stage embryo which has prominent trigeminal ganglion primordia.

The primitive streak develops within an embryonal area as part of the superficial wall of the yolk-sac, a feature also shared with marsupials, birds and reptiles. The major expansion of extra-embryonic membranes occurs after the formation of the primitive streak. A column of fine yolk spheres extends from beneath the embryonic blastodisc towards the centre of a yolky vitellus, as seen in birds. A feature of the pre-embryonic development of the platypus is the incomplete meroblastic cleavage. This paper presents new observations on developmental stages from the Hill collection, which allow for a more complete description of pre-hatching development. Hill and now housed in the Hubrecht International Embryological Laboratory, Utrecht, contributes a major source of specimens. Information on the pre-hatching development of the platypus, Ornithorhynchus anatinus, is reliant on a small number of specimens, whose precise age is unknown.

Platypus size