Iks

Invaginating mesoderm cells

Hypoblast

Figure 4.3 A. Dorsal side of the germ disc from a 16-day embryo indicating the movement of surface epiblast cells (solid black lines) through the primitive streak and node and the subsequent migration of cells between the hypoblast and epiblast (broken lines). B. Cross section through the cranial region of the streak at 15 days showing invagination of epiblast cells. The first cells to move inward displace the hypoblast to create the definitive endoderm. Once definitive endoderm is established, inwardly moving epiblast forms mesoderm. C. Scanning electron micrograph through the primitive streak of a mouse embryo showing migration of epiblast (eb) cells. The node region appears as a shallow pit (arrow). D. Higher magnification of the section in C.

in the epiblast, the neurenteric canal temporarily connects the amniotic and yolk sac cavities (Fig. 4.4A).

The cloacal membrane is formed at the caudal end of the embryonic disc (Fig. 4.3A). This membrane, which is similar in structure to the buccopharyngeal membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm. When the cloacal membrane appears, the posterior

Neurenteric Canal
Figure 4.3—Continued.

Primitive pit and neurenteric canal

Amnion

Wall of yolk sac -t

Primitive pit and neurenteric canal

Amnion

Wall of yolk sac -t

Connecting stalk

Allantois

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