The formation of the embryo commences with the appearance of the medullary plate, the sides of which soon grow up to form the medullary folds. The medullary groove is developed anteriorly before any trace of it is visible behind. In a general way the closure of the groove takes place as in Birds, but the anterior part of the body is very early folded off, sinks into the yolk, and becomes covered over by the amnion as by a hood (figs. 127 and 129). All this takes place before the closure of the medullary canal; and the changes of this part are quite concealed from view. [FIG. 129. SURFACE VIEW OF A YOUNG EMBRYO OF LACERTA MURALIS. _am._ amnion; _pr._ primitive streak.] The closure of the medullary canal commences in the neck, and extends forwards and backwards; and the whole region of the brain becomes closed in, while the groove is still largely open behind. The later stages in the development of the Lacertilian embryo do not require a detailed description, as they present the closest analogy with those already described for Aves. The embryo soon turns on to its left side; and then, becoming continuously folded off from the yolk, passes through the series of changes of form with which the reader is already familiar. An advanced embryo is represented in fig. 130. The early development and great length of the tail, which is spirally coiled on the ventral surface, is a special feature to which the attention of the reader may be called. _Embryonic Membranes and Yolk-Sack._ The early development of the cephalic portion of the amnion has already been alluded to. The first traces of it become apparent while the medullary groove is still extremely shallow. The medullary plate in the region of the head forms an axial strip of a thickish plate of epiblast. The edge of this plate coincides with the line of the amniotic fold, and as this fold rises up the two sides of the plate become bent over the embryo and give rise to the inner limb of the amnion or amnion proper. The section (fig. 127), representing the origin of the amniotic hood of the head, shews very well how the space between the two limbs of the amnion is continuous with the body cavity. The amnion very early completely encloses the embryo (fig. 128 A and B), and its external limb or serous membrane, after separating from the true amnion, soon approaches and fuses with the vitelline membrane. [FIG. 130. ADVANCED EMBRYO OF LACERTA MURALIS AS AN OPAQUE OBJECT[73]. The embryo was 7 mm. in length in the curled up state. _fb._ fore-brain; _mb._ mid-brain; _cb._ cerebellum; _au._ auditory vesicle (closed); _ol._ olfactory pit; _md._ mandible; _hy._ hyoid arch; _br._ branchial arches; _fl._ fore-limb; _hl._ hind-limb.] [73] This figure was drawn for me by Professor Haddon.