I am reading Griffiths' paper Periods of Integrals on Algebraic Manifolds I, and in section 2 I met some problems.
I wish that I could get some help here. My problem is that I cannot understand formula (2.19) and (2.20) on page $584$. I think that I know how to get the graph of a given morphism, but I cannot understand how the author could get the concrete formula (2.19) for its homology class. Thank you very much!
Here is the paper: http://publications.ias.edu/sites/default/files/periodsofintegralI68.pdf