'twixt-brainn. = 'tween-brain: see 'tweenprep. Compounds 1.
ΚΠ
1878 F. J. Bell & E. R. Lankester tr. C. Gegenbaur Elements Compar. Anat. 503 These primitive cerebral vesicles give rise to new segments... The first is known as the Fore-brain or Prosencephalon; the next as the Twixt-brain or Thalamencephalon.