EARLY DEVELOPMENT AND EMBRYOLOGY OF THE PLATYPUS

Information on the pre-hatching development of the platypus, Ornithorh ynchus anatinus, is reliant on a small number of specimens, whose prec ise age is unknown. Material collected for J. P. Hill and now housed i n the Hubrecht International Embryological Laboratory, Utrecht, contri butes a major source of specimens. This paper presents new observation s on developmental stages from the Hill collection, which allow for a more complete description of pre-hatching development. A feature of th e pre-embryonic development of the platypus is the incomplete meroblas tic cleavage. A column of fine yolk spheres extends from beneath the e mbryonic blastodisc towards the centre of a yolky vitellus, as seen in birds. The major expansion of extra-embryonic membranes occurs after the formation of the primitive streak. The primitive streak develops w ithin an embryonal area as part of the superficial wall of the yolk-sa c, a feature also shared with marsupials, birds and reptiles. The full -term, subspheroidal, intrauterine egg of the platypus has a major axi s of about 17 mm and contains a flat, 19-20 somite, neurula-stage embr yo which has prominent trigeminal ganglion primordia. The embryo at th is stage is in a period of rapid modelling of the major early organ pr imordia of the nervous system, cardiovascular system, excretory system , and somite-derived components of the body wall. Soon after laying, f ive primary brain vesicles are present, the trigeminal ganglia CN5 as well as CN7, CN8, CN9, CN10, CN11 and CN12 are well developed. The ali mentary system has an expanded stomach, pancreatic primordia and a gal l bladder. Mesonephric tubules are associated with patent mesonephric duels, which empty laterally into the cloaca. Extra-embryonic membrane s at this stage show an extensive chorioamniotic connection that exten ds through the greater parr of the caudal half of fused amniotic folds . The vascularized yolk-sac consists of a superficial yolk-sac omphalo pleura and a deep yolk-sac splanchnopleure. The non-vascularized yolk- sac comprises one-quarter of the abembryonal pole. Some distinctive mo notreme features have developed by the mid-incubation period. The head is bent at an acute angle to the main body axis. The blunt upturned s nout marks the site of the future oscaruncle and on the maxilla there is a median primordial papilla representing the egg tooth. The eye is open with a partly pigmented retinal ring. The forelimbs have partly s eparated digits, and the hindfeet are paddles. Just before hatching th e upturned snout contains an oscaruncle and a sharp recurved median eg g tooth. Forelimbs are pronated with separate digits possessing claw p rimordia. Portions of the highly vascularized extra-embryonic membrane s are attached to the umbilical region and the flattened vesicular all antois has a distal legion fused with the chorion. Prominent features of the hatchling are the presence of a bluntly conical oscaruncle and a translucent, horn-like egg tooth. These structures are thought to en able the hatchling to extricate itself from the egg shell. At hatching , the forelimbs exhibit clawed digits and are capable of digitopalmar prehension. Hindlimbs are still paddles with digital rays. A prominent yolk-sac navel is present. The newly hatched platypus has all externa l form similar to that of a new-born marsupial. The early development of the platypus has many major differences to the developmental sequen ce for humans, which has been categorized by the use of Carnegie Stage s. The rate of somitogenesis of the platypus is faster in relation to the central nervous system morphogenesis than seen in humans, and the size of the early platypus embryonal area is massive in relation to th at of humans. The unique morphology and function of extra-embryonic me mbranes in the platypus defies comparative staging with human developm ent. Structures adapted for altricial survival of the platypus hatchli ng require the acquisition of functional competence at an earlier stag e of organogenesis than seen in eutherians, although they are reminisc ent of those found in new-born marsupials.