Embryonic Development in Man. 3Y3 



Description of the Embryo. 



The sections of the embryo have been accurately drawn under 

 my direction by Miss Grace Amadon, and are shown in Plates I to 

 IV, Figs. 1 to 22. The embryo is first seen in section 164 (Fig. 1 of 

 the embryo), and the ectoderm cells of the embryonic shield were 

 last encountered in section 149. Hence the whole length of the 

 embryonic shield proper is IG by 7 microns equals 112 microns. 

 However, the mesoderm extends as a thickened mass beyond the ecto- 

 dermal limit of the embryonic shield from sections 150 to 143, or 

 over a distance of 8 sections, equal in all to 56 microns. We have 

 here what appears to be an extension of the shield-mesoderm beyond 

 the shield ectoderm and entoderm — a mesoderm ''Vorhof." Includ- 

 ing the mesoderm "Vorhof," the embryonic shield extends through 

 twenty-two sections; its whole length therefore is 22 by 7 microns 

 equals 154 microns. 



The first section (164) has hit the embryo in a tangential man- 

 ner, and shows only a single layer of cells. The three following 

 sections (Figs. 2 to 4) consist of an inner layer of ectoderm and 

 an outer layer of mesoderm. In the next section (Fig. 5) a 

 delicate strand of cells is clearly seen between the ectoderm and 

 mesoderm. This must be interpreted as a subdivision of the meso- 

 derm. It continues into the eighth section, in which the entoderm 

 first appears. The entoderm is represented by a small group of 

 cells between the two layers of mesoderm, seen on the upper side of 

 Fig. 8. In Fig. 9 the entoderm has expanded so that the cavity 

 of the yolk sac now appears. It lies in the upper half of the 

 figure. 



The embryo is anchored to the chorion by an allantoic stalk, com- 

 posed externally of mesoderm and traversed by a rather slender, 

 somewhat curved canal of entoderm cells (Figs. 9 to 14). The con- 

 nection between the entoderm of the allantois and of the yolk sac 

 presumably occurred in sections 156 and 155, but the continuity 

 of the entoderm has been destroyed. 



Around the allantoic stalk (Figs. 10 to 13) where its mesoderm 

 is continuous with the yolk sac mesoderm, there are found some 

 solid and some open circular masses of mesoderm cells. The open 



