EMBRYONIC-DEVELOPMENT OF PINEAL COMPLEX O F PLEURODELES WALTL (AMPHIBIA, URODELA) - PRESENCE OF PARAPINEAL ORGAN

The pineal complex of Pleurodeles waltl, considered as an evoluated Ur odela, was bipart. Contrary as it was thought, a pineal organ and an a nterior parapineal organ were both perceptible. They were joined one b ehind the other at the top of the diencephalon. The distinction betwee n these two parts could only be made by an embryonic study. Four main phases were perceptible during the organogenesis. During the first pha se (stages 30 to 38), the pineal complex was putting into position. An evagination of the roof of the diencephalon gave the primordium of th e complex. This primordium was rapidly divided in two holow parts. Fro m the cytological point of view, three cellular standarts are differen ciated during this phase : the supportive cells, the neuronal cells an d the typical photoreceptors Lining the lumens. The individualization and differential growth of the two organs took place during the second phase (stages 39 to 47). The lumen of the anterior pineal organ was f illed up and the pineal organ was growing longer and wider. The third phase (stages 48 to 50) was characterized by the acceleration of the g rowth. This growth was fundamentally made owing to the extension of th e caudal part. During the fourth phase (stage 51 to metamorphosis), th e pineal complex was being flattened and elongated. An important compa rtmentalization of the pineal lumen was present. The photoreceptors we re transformed. This was the adult structure. A comparison with the ot her Amphibians could also be done thanks to the table of the pineal de velopment in Pleurodeles waltl. The adult structure of the pineal comp lex in Hynobius, an archaic Urodela, was like the structure of the sec ond phase. The pineal evolution in Urodela seemed to be characterized by the filling up of the lumens. At the first embryonic stages, Anoura were like Urodela The pineal evolution of Anoura was nevertheless dif ferent because the parapineal and the pineal organ were well separated in adult. The adult pineal pattern of Gymnophiona was more different than the other pineal complex. It only included a pineal organ wich wa s not very developped. It looked like the one of Urodela in the middle of the first phase. This represented an other pole of pineal evolutio n.