HUMAN EMBRYO IN THE FOURTH STAGE. 



135 



than elsewhere. Along its median line runs the deep, narrow, dorsal groove which 

 at its caudal end widens out and disappears. Just behind it is the opening of the 

 relatively large neurenteric canal, behind which again follows a remnant of the 

 primitive groove. A transverse section a little in front of the middle of the em- 

 bryo is shown in figure 36. The ectoderm, ek, is very much thickened to con- 

 stitute the medullary plate; the narrow central longitudinal furrow, /, mentioned 



Md.gr. 



Neu. c. 



Cho. 



FIG. 79. RECONSTRUCTION OF A HUMAN EMBRYO i . 54 MM. LONG. THE AMNION HAS BEEN OPENED 



TO SHOW THE DORSAL SURFACE OF THE EMBRYO. 



Yk, Yolk-sac. Am, Amnion. Md.gr, Medullary groove. Neu.c, Neurenteric canal. Pr.gr, Primitive 

 groove, b.s, Body-stalk. Cho, Chorion. (After Count Spec.) 



above is very noticeable. Outside of the embryo the ectoderm is reflected on to the 

 amnion, ct, over the back of the embryo. The entoderm is a thin layer of cells in 

 the center of which the notochordal band ch can be distinguished. In sections 

 near the neurenteric canal the notochord is better marked, being there much 

 thicker than the remaining entoderm, The mesoderm, me, is a distinct layer, 

 although, as other sections show, it is fused in the median line of the primitive 

 streak behind the neurenteric canal with both ectoderm and entoderm. Although 

 the extra-embryonic coelom is fully developed, that of the embryo is present as a 

 small fissure, p, only. Figure 81 is a section passing through the neurenteric canal, 

 and shows, therefore, the amnion, am, the thickened medullary plate, e, of the em- 

 bryo, and the large yolk-sac, d. The yolk-sac is formed, of course, of splanchno- 



