36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



away from the cavity of the enteron. The body cavity (be) is here 

 still unenclosed, and its walls, the somatic stalk, are cut off close to 

 the body of the embryo. The heart (lit), the most conspicuous 

 feature of this section, is nearly as large in cross-section as all the 

 rest of the embryo. As seen in such a section it is entirely detached 

 from the body of the embryo, and in this particular case has about 

 the shape of the human stomach. The mesoblastic portion of its 

 wall (mes') is of very irregular thickness; it forms a dense layer 

 entirely around the outside, except for the pointed dorsal region, 

 and is especially thick along the ventral margin, where it is thrown 

 into well-marked folds, the heavy muscle columns. Lining the 

 cavity of the heart is the membranous endothelium {en'), and be- 

 tween this and the dense outer wall just described is a loose reticular 

 tissue with but few nuclei. 



As the series is followed toward the tail the sections diminish in 

 size until, at a point about one-third the embryo length from the 

 posterior end, they are of scarcely one-fourth the area of the sec- 

 tions through the region of the hindbrain. 



Figure 130? is about one hundred and twenty-five sections pos- 

 terior to figure 13c. Although not so small as the sections that 

 follow it, this section is considerably smaller in area than the one 

 last described. The amnion (a), which was not represented in the 

 last three figures, is very evident here. The spinal cord (sc) is 

 considerably smaller here than in the preceding figure, while the 

 notochord (nt) is not only relatively but actually larger than in the 

 more anterior regions. Beneath the notochord is the aorta (ao) y 

 now a single large vessel. The mesoblast on each side of the body 

 is here differentiated into a distinct muscle plate (mp). These 

 muscle plates have very much the appearance of the thickened ecto- 

 derm seen in the younger stages of development. At about its 

 middle region (i. e., at the end of the reference line ec) each muscle 

 plate is separated from the overlying ectoderm by an empty space ; 

 this space is still more marked in some other series. Ventral to the 

 aorta, and supported by a well marked though still thick mesentery 

 (ms), is the intestine. It is a small, nearly cylindrical tube with 

 thick walls ; the splanchnic mesoblast which surrounds it is more 

 dense than the general mass of mesoblast ; it was somewhat torn 

 in the section and is so represented in the figure. The urinar) 

 organs have made considerable progress since the last stage. In 

 the figure under discussion they are seen as a group of tubules on 

 either side of the aorta. The tubule most distant from the middle 

 line, on each side, is the Wolffian duct {wd). It extends through 

 the posterior two-thirds of the embrvo and varies in diameter at 



