Primary neurulation and secondary neurulation are important developmental processes and have been described in many models. In the chick, there does not exist a clear distinction as to when primary neurulation ends and secondary neurulation begins; the lower spinal cord has been described as junctional neurulation, whereby ingression and accretion accompany the process of defining the area which straddles primary and secondary neurulation and is therefore thought to somehow represent human thoracolumbar spina bifida .