Final answer:
The layer of cartilage that remains between the epiphysis and diaphysis until bone growth in length is complete is known as the epiphyseal plate.
Step-by-step explanation:
Until bone growth in length is complete, a layer of cartilage known as the epiphyseal plate, or growth plate, remains between the epiphysis and diaphysis. This plate allows for the lengthening of long bones during childhood and adolescence. Chondrocytes on the epiphyseal side of the epiphyseal plate divide and facilitate growth. Over time, these chondrocytes cease proliferation and cartilage is replaced by bone, signifying the end of longitudinal growth with the formation of the epiphyseal line.