Cohort effects and period effects are shared by cohorts.

It is shared by birth cohorts.  It is used in social science to defined variations in the characteristics of an area of study such as the incidence of a characteristic or the age at onset, such as year of birth, or year of exposure to radiation, ,  over time among individuals who are defined by some shared brain experience or common life experience. 

b tumor cells have a cell wall instead of a membrane

explanation:

tumor cells different from normal cells because tumor cells have a cell wall instead of a membrane.