Instead , it begins approximately at the level of the fourth somite at Carnegie stage 9 ( around Embryonic day 20 in humans ) .
The folding of the neural tube to form an actual tube does not occur all at once .
It begins approximately at the level of the fourth somite at Carnegie stage 9 ( around Embryonic day 20 in humans ) .
The folding of the neural tube to form an actual tube does not occur all at once .