426 OUTLINES OF CHORD ATE DEVELOPMENT 



of mesoderm, and therefore composed of ectoderm and endo- 

 derm only. This mesoderm-free region is called the proamnion 

 (Fig. 176). The amnionic folds appear, toward the end of the 

 ninth day, between the embryo and the horse-shoe shaped 

 implantation area described above. At this stage of develop- 

 ment the embryo is well established, its head- and tail-folds are 

 formed, and the head is beginning to enlarge. In the wall of 



Cho. 



FIG. 177. Transverse section through the rabbit embryo of eight days and 

 two hours. From Minot (Laboratory Text-book of Embryology). Am, Am- 

 nion; Ao, lateral dorsal aorta; Ch, notochord; Cho, chorion; Cce, coalom; Ent, 

 endoderm; Md, medullary tube (nerve cord); Seg, myotorne; Som. somato- 

 pleure; Spl, splanchnopleure. 



the blastodermic vesicle the endoderm has extended nearly or 

 quite completely around the inside of the vesicle (tropho blast), 

 while the mesoderm with its exocoelom extends through only 

 the upper third of the vesicle, which is nearly as far as it ever 

 goes in the rabbit. 



The tail-fold of the amnion appears first, composed of ecto- 

 derm and somatic mesoderm of the extra-embryonic region, 

 and therefore containing an extension of the exocoelom; the 

 mesodermal layer is unusually thick in the tail-fold. Later- 

 ally folds soon appear as anterior extensions of the extremities 

 of the tail-fold. The head of the embryo rapidly enlarges, and 

 as it sinks down into the proamnionic region this flows up to or 



