| Abstract: | SYNOPSIS. The protargol technic was used in a study of the development of oral, cirral, and dorsal primordia of Urostyla weissei fixed during division, reorganization, and regeneration following transection at different levels. While the course of development is similar in all situations, differences were observed in the way in which some primordia are initiaily formed. The primordium of the new AZM always appears posterior to the old AZM. It develops into an entire new membranellar band in dividing cells and in opimers (posterior fragments from equatorial transections), while it eventually joins with a portion of the old AZM in reorganizers, promers (anterior fragments from equatorial transections) and “large opimers” (cells whose anterior tip has been cut off). The UM-primordium of proters is derived from disaggregation of the kinetosomes of the 2 old UM's, that of opisthes and opimers is formed “de novo” to the right of the AZM-primordium, while the UM-primordium of reorganizers, promers, and “large opimers” is of composite origin, partly “de novo” and partly from the old UM's. The UM primordium differentiates into the new UM's and the 1st frontal cirrus. The primordia of the remaining frontal, ventral, transversal (F-V-T) and marginal cirri originate as “streaks” of cilia, most of which are derived from re-alignment of the constituent cilia of certain pre-existing cirri. New cirri differendiate from the streaks, and replace the remaining old cirri. The streaks are formed similarly in all developmental situations, except for the 1st 3 F-V-T streaks. In proters, reorganizers, and promers, these originate from the posterior 3 frontal cirri, while in opisthes and opimers they are formed “de novo” to the right of the UM-primordium. In the “large opimers” these streaks are formed “de novo” behind the 1st 3 frontal cirri, in spite of the continued presence of these cirri at the anterior tip of the fragments. The site of formation of these streaks thus appears to be determined by an anteriorposterior gradient, rather than by any preformed cortical structure. The new dorsal bristle rows I to III develop from the proliferation of portions of the old rows, while rows IV and V originate from short kineties formed “de novo” on the right margin. New caudal cirri differentiate at the posterior ends of the new rows I to III. The numbers of ventral cirral rows and transversal cirri are variable; these variations are correlated, and related to variations in numbers of developing streaks. A survey of hypotrich developmental patterns revealed extensive parallels, especially in the sites of appearance of primordia. The primordium site appears to be a more constant feature of cortical development than is the “source” of ciliary units. It is concluded that sites of primordia are determined by cellular gradients, with competent preformed structures being utilized if they are appropriately positioned within these gradients. |
