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Theoretical models of normal type II Cepheids in the period range 1-10 days have been constructed with the use of the new Carson opacities. The various features of the computed light and velocity curves are found to agree in detail with those actually observed. For example, the Hertzsprung progression of the light curves shows up distinctly in the approximate period range 1-3 days. Masses of observed type II Cepheids, deduced both from the phase of the Hertzsprung bump in the light curves and from the theoretical pulsation constants, are consistent with each other, and average, formally, 0.59 ± 0.03 M<SUB>sun</SUB> for globular cluster members and 0.54 ± 0.01 M<SUB>sun</SUB> for the archetypal field variable BL Herculis. These masses also agree closely with masses determined directly from atmospheric analyses and indirectly from stellar-evolution theory. From the location of the observed blue edge of the instability strip in the H-R diagram, the helium abundance in the pulsating layers is inferred to be 0.25 ≤ Y ≤ 0.50 and from a comparison of the bump masses and the pulsation-constant masses, it is determined to be Y = 0.31 ± 0.08. This amount of helium is in satisfactory agreement with spectroscopic and evolutionary data for these stars. <P />If the Cox-Stewart opacities are adopted instead of the Carson opacities, the bump masses turn out to be, at least formally, too small compared to the pulsation-constant masses and to the expected evolutionary masses of the helium cores of these stars, for any realistic helium abundance.