THE EARLY DEVELOPMENT OF THE MAMMAL 401 



A slightly later stage is shown in sagittal section in Fig. 161, E. 

 At this time the embryo (Graf Spec's embryo Gle) measures 

 1.54 mm. in length by about 0.7 mm. in greatest breadth, while 

 the entire vesicle ("ovum") is approximately 10 mm. in di- 

 ameter. Its dorsal surface is occupied by the large neural 

 plate, with a distinct neural groove. Toward the posterior end 



FIG. 162. Young human embryos. After Keibel and Elze. A. Keibel and 

 Elze's Embryo Klb, X 25. B. Kollmann's Embryo Bulle, X 20. For de- 

 scription, see text. 



is the small neurenteric canal, behind which may be seen a short 

 primitive streak and groove. A short thick body stalk attaches 

 the embryo to the chorionic wall of the blastodermic vesicle. 

 The wall of the ovoid yolk-sac, whose diameters are about 

 equal to the length of the whole embryo, is already quite 



after Graf von Spee. In all the figures the anterior end is toward the left 

 Black, embryonic ectoderm; heavy stipples, trophoblast and trophoderm; light 

 stipples, endoderm; oblique ruling, mesoderm. A. Hypothetical early stage; 

 mesoderm a solid mass. B. Amnionic cavity and wide exoccelom established; 

 endoderm limited to a small vesicle beneath the embryonic ectoderm. The 

 exoccelom in reality contains scattered mesenchyme cells. C. Blastodermic vesi- 

 cle enlarged and covered with trophodermic villi, into which the mesoderm is ex- 

 tending. Endodermic vesicle (yolk-sac) very small (stage of Peter's ovum). 

 D. Embryonic portion only, of an older vesicle showing the neurenteric canal, 

 primitive streak (in the plane of the section), and body stalk. The mesoderm 

 of the yolk-sac is becoming vascular. E. Sagittal section through the human 

 embryo of 1.54 mm. (Graf von Spec's embryo Gle). a, Amnionic cavity; al, 

 allantois; am, amnion; B, body stalk; ch, chorion; e, exoccelom; h, heart region; 

 nc, neurenteric canal; V, chorionic villi; Y, yolk-sac. 



