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.
The first development of the allantois as a diverticulum of the hypoblast covered by splanchnic mesoblast, at the apparent posterior end of the primitive streak, has been described on p. [207]. The allantois continues for some time to point directly backwards; but gradually assumes a more ventral direction; and, as it increases in size, extends into the space between the serous membrane and amnion, eventually to form a large, highly vascular, flattened sack immediately below the serous membrane.