It forms from a small part of the front of the primitive neural tube, the structure that becomes the nervous system (brain and spinal cord) of the adult.
(It; forms; from a small part of the front of the primitive neural tube) (the structure; becomes; the nervous system (brain and spinal cord) of the adult)
It the structure
forms becomes
from a small part of the front of the primitive neural tube
the nervous system (brain and spinal cord) of the adult