The relationships between genome size and various cell morphometric parameters have been assayed in 357 species of Vertebrates, in order to verify the existence and significance of the so-called "nucleotypic effect" in this subphylum. The results obtained clearly manifest a significant relationship between the increase in genome size and that in nuclear volume, nuclear surface, cell volume and cell surface. A precise correlation is also observed between the increase in DNA content and the decrease in the surface/volume ratios of the nucleus and the cell. Other parameters, such as the nucleoplasmic index and DNA concentration, though showing a slight increase with increasing genome size, have values rather homogeneous in each Vertebrate group. These results have allowed some interesting speculations on various problems; for example, the mechanisms through which genome size can influence the cell size; the influence of the DNA content and cell morphometric parameters on functional level of the cell and the organism; the importance of the nucleotypic effect in the adaptation to the environment of the various Vertebrate groups. From this study it seems possible to make the following conclusions: 1) in Vertebrates, genome size would exert a real nucleotypic influence on cell size; 2) genome sizes and cell morphometric parameters seem to be involved in the regulation of cell metabolism; 3) the regulation of some morphometric parameters depends strictly and automatically on the DNA amount or on other morphometric parameters. The regulation of others, instead, depends on the interaction of different factors, which do not always act synergically; 4) the nucleotypic effect seems to have different distribution and importance in Anamniotes and Amniotes.