ABSTRACT
The Anlage of the pelvic fins appears in Salmo 2 weeks after the fecundation day, at the level of the somites 23-26. The mesoderm has a double origin and seems to differ in this regard from the hind limb of most of the amniotes.
The ‘initial mesenchyme’ comes from the local proliferation of the somatopleura. It will give essentially the skeletal components of dermal origin (actinotrichia and lepidotrichia). The ‘secondary mesenchyme’ is obtained by the dispersion of four ventral somitic processes that have reached and entered the initial mesenchyme blastema. The secondary mesenchyme will probably give the muscles and also the endoskeleton of the fin. The origin of the girdle is not clear. When the setting of the initial mesenchyme begins the epithelium that covers the embryo is already differentiated in an epiderm with numerous mucous cells resting on a visible basement membrane. At the apex of the pelvic bud, a localized and transitory thickening of the epiderm is produced by the increase in the height of the basal stratum. We call this structure by the name of ‘pseudo apical cap’ to stress the fact that it must be distinguished from the ‘apical cap’ described for the limb bud of amniotes.
So, the morphogenesis of the pelvic fins of Salmo shows some important particularities in the epiderm as well as in the mesoderm.
Comme le suggère Bouvet (1968), nous utiliserons le terme d’épiderme et non d’ectoderme, puisqu’il y a déjà d’importantes différentiations cellulaires, en particulier des cellules muqueuses abondantes.
