The results of numerical simulations aimed at studying large-scale morphological features in the structure of gaseous galaxies disks are presented. The behaviour of a gaseous disk in an external gravitational potential that does not contain spiral modes is analyzed. The formation characteristics of spiral substructures induced by tidal disturbances, which are produced in the interactions with companion galaxies is revealed. Slowly rotating two-armed spiral waves are generated. The question of the duration of arising spirals is discussed. The appearance of a global polygonal pattern in a differentially rotating gaseous disk is demonstrated. All the main geometrical properties of observed rows in spiral galaxies are satisfactorily reproduced.