Fig. 2. Isolation of the allantois and chorion. (A) Schematic diagram illustrating isolation of the allantois (al) with a glass microcapillary pipette. Several cell layers were trimmed from the base with a glass scalpel. am, amnion; ys, yolk sac. (B) Chorion (ch) isolation. Endoderm was labeled by submerging the conceptus in ConA 594 (green) and injecting ConA 488 (red) into the exocoelomic cavity (ecc) to label the mesoderm that lines the cavity. bi, blood island; epc, ectoplacental cavity. Two incisions (dotted lines) were made to isolate the chorionic plate, which is shown in the right hand panels. We then trimmed visceral endoderm (ve, green) away from the chorion (ch, red) with a glass scalpel. After 24 hours of explant culture, chorions isolated in this way had no visible traces of green fluorescent ve-derived cells (Fig. 4A,B). (C) Isolation of anterior embryonic ectoderm (control for Table 1). The endoderm was labeled with ConA 594 (green). Three cuts were made with glass scalpels: 1, a transverse cut proximal to the amnion; 2, a sagittal cut to isolate the anterior portion of the embryo; 3, the embryonic endoderm and mesoderm were removed from the ectoderm with pancreatin/trypsin digest followed by excision with a glass scalpel.