▁original ▁verte bra e ▁might ▁be ▁most ▁expected , ▁the ▁sk ull ▁is ▁an ▁un segment ed ▁gr ist ly ▁brain - box , ▁and ▁that ▁in ▁higher ▁forms , ▁the ▁verte br al ▁nature ▁of ▁the ▁sk ull ▁cannot ▁be ▁maintained , ▁since ▁many ▁of ▁the ▁b ones , ▁not ably ▁those ▁along ▁the ▁top ▁of ▁the ▁sk ull , ▁arise ▁in ▁the ▁skin . ▁ ▁In ▁ 1 8 5 8 , ▁the ▁phys ician ▁Ernst ▁H ae ck el ▁studied ▁under ▁Gegen ba ur ▁at ▁J ena , ▁receiving ▁a ▁doctor ate ▁in ▁zo ology ▁( after ▁his ▁medical ▁degree ), ▁and ▁became ▁a ▁professor ▁at ▁the ▁same ▁institution , ▁the ▁University ▁of ▁J ena ▁( see : ▁Ernst ▁H ae ck el ). ▁ ▁Ernst ▁H ae ck el ▁expanded ▁on ▁the ▁ideas ▁of ▁Gegen ba ur ▁while ▁advoc ating ▁the ▁concepts ▁of ▁Charles ▁Dar win . ▁ ▁In ▁ 1 8 6 1 , ▁he ▁published ▁" U e ber ▁den ▁Bau ▁und ▁die ▁Ent wick el ung ▁der ▁Wir bel th ier - E ier ▁mit ▁part ielle er ▁D ot ter bildung " ▁(" Proof ▁that ▁the ▁ov um ▁is ▁un ic ell ular ▁in ▁all ▁verte br ates ", ▁Arch . ▁An at . ▁Phys ., ▁ 1 8 6 1 . 8 : ▁ 4 6 1 - 5 2 9 ), ▁a ▁fundamental ▁proof ▁in ▁emb ry ology . ▁ ▁Gegen ba ur ▁learned ▁techniques ▁as ▁a ▁student ▁of ▁Albert ▁von ▁K öl