In the study of the chondrocrania of embryonic mammals, it is probably true to say that no form at the maximum stage of development of the cartilage has been more thoroughly investigated than the rabbit, by Voit. His material, however, consisted of well-developed stages only, of 43, 45, and 70 mm. greatest length ; and apart from a few remarks by Noordenbos, the earliest stages of development of the skull of the rabbit remained unknown. This was an unfortunate gap, for while what may be called the morphology of the chondrocranium is now, thanks to the work of Gaupp and Fawcett among others, well understood, the embryology of the chondrocranium is still mysterious. In four forms only among the mammals, viz., mole (Noordenbos) ; cat (Terry) ; Tatusia (Fawcett) ; and man (various authors); can it be said that the earliest appearance of the various cartilaginous structures has been followed. It seemed particularly desirable, therefore, to extend the present excellent knowledge of the late skull of the rabbit back to the earliest stages of its development. The authors owe the incentive to this work to Prof. J. P. Hill, F.R.S., to whom they wish to express their thanks for encouragement and help freely given in many ways. They also desire to acknowledge their debt to Prof. E. S. Goodrich, F.R.S., whose interest in the work has been of great assistance to the authors.