Epibole

Epibole. The cells which form the primitive embryonic stage, known as the blastosphere (q.v.), are usually different in size at the two ends. The smaller cells form the external layer or epiblast by two different methods. Either they grow round the larger "endoderm" cells - a process known as "epibole" - or the endoderm cells sink towards the centre of the embryo, a process known as "embole."