606 J. T. PATTERSON 



time; thus exposing the entoderm to the uterine cavity. The 

 entoderm is connected at the margin to a thickened portion of 

 the chorionic ectoderm, which is recognized as the Trager (2V.). 



The mesothelium which lines the extraembryonic cavity (E. 

 E. B. (7.), has undergone no important changes; but there has 

 been organized just beneath it a distinct Trager epithelium 

 (Tr. Ep.), which in its relation to the uterine mucosa is particu- 

 larly clear in the photograph (fig. 64). 



The most important changes involve the ectodermal vesicle, 

 and chief among these is the one affecting the organization of 

 the embryonic primordia. On account of the inversion of germ 

 layers, the lower side of each embryonic primordium is upper- 

 most in the figure, that is, the lower side of the future embryo 

 will be on the outside of the ectodermal vesicle. The section 

 under discussion shows the rudiments of Embryos I and II, which 

 are cut across at a slightly oblique angle (fig. 2). Each embry- 

 onic spot is characterized by (1) a fusion between the ectoderm 

 and the entoderm, and (2) by a rather shallow depression, or 

 primitive groove, which is situated on the lower or inner side of 

 the embryonic ectoderm (fig. 23, P. G.). Between the two em- 

 bryos the entoderm is entirely free from that portion of the ecto- 

 derm which joins the adjacent sides of the two embryonic rudi- 

 ments. The cavity lying below the embryos is the bay of the 

 original right-hand primary bud, and is a portion of the general 

 amniotic cavity. The thin lower wall of this cavity is, there- 

 fore, the ectodermal layer, which, together with the adjacent me- 

 sodermal epithelium, constitutes the true amnion (fig. 23, Am.). 



The primitive groove of each embryo measured about 0.095 

 mm., and is rather shallow throughout the greater part of its 

 length. There is, however, a pit-like depression at one point 

 which undoubtedly corresponds to a primitive pit. This is es- 

 pecially distinct in Embryo I, in which it lies about 0.032 mm., 

 from the anterior end of the groove (fig. 24, P. P.). 



Beneath the primitive groove there is the typically early prim- 

 itive streak region, from which cells are being proliferated later- 

 ally to form the true embryonic mesoderm (fig. 24, E. Mes.). 

 Throughout the extent of the primitive streak there is no 



