388 OUTLINES OF CHORD ATE DEVELOPMENT 



opossum, the ectoderm moves up irregularly, gradually pushing 

 away the trophoblast cells (Figs. 152, 153, 157). (3) In the 

 rabbit, shrew, and probably in the dog, the trophoblast remains 

 for a time continued as a very thin layer (Ranker's layer) over 



FIG. 153. Transverse sections through the early blastodermic vesicle of the 

 hedgehog, Erinaceus. After Hubrecht. A. Embryonic ectoderm forming a solid 

 mass. Endoderm lining the entire trophoblast wall. Trophoblast becoming 

 trophodermic. B. Amnionic cavity established. Trophoderm formed com- 

 pletely around the blastodermic vesicle, a, Amnionic cavity; c, ectoderm; 

 d, trophoderm; n, endoderm; s, embryonic shield; t, trophoblast. 



FIG. 154. A. Section through part of the blastodermic vesicle of a six-day 

 rabbit. From Quain's Anatomy, after Van Beneden. B. Transverse section 

 through the embryonic shield of a dog of unknown age (between eleven and fifteen 

 days). After Bonnet, a, Trophoblast (in A, Rauber's layer); b, embryonic 

 ectoderm; c, endoderm; s, embryonic shield (embryonic ectoderm); x, space in 

 embryonic shield. 



the surface of the ectoderm of the embryonic region. Appa- 

 rently the thin cells of the trophoblast finally disappear, leaving 

 the ectoderm upon the surface, but it is possible that they mingle 



