56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



connection of this nerve with the near-by gill cleft is seen. Above 

 the paired aortas (ao) the sympathetic nerves (sy) will be noticed. 

 The mesoblast surrounding the spinal cord (sc) and notochord 

 (nt) is distinctly condensed (more so than the figure shows) to 

 form what may be called the centrum (c) and neural arch (tun of 

 the vertebrae. The arch, owing to the slight obliquity of the section, 

 shows here only on one side. The spinal cord is not yet completely 

 enclosed by the neural arches. The muscle plates ( mp ) are in close 

 connection with the rudiments of the vertebrae just mentioned. The 

 spinal cord (sc) is here differentiated into three areas — a dense, 

 deeply stained area immediately around the neurocoel ; a less dense 

 area of cells surrounding the inner area and extending ventralward 

 as a rounded projection on each side; and an outer layer, with few 

 or no nuclei, surrounding the inner two layers except on the dorsal 

 side. 



In figure 20i the size and complexity of the figure are due, it will 

 be easily understood, to the fact that the plane of the section passed 

 through the curve of the body, thus practically cutting the embryo in 

 two regions — an anterior, where the lungs (lu) and liver (li) are 

 seen, and a posterior, where the Wolffian bodies (wt) are present. 

 The spinal cord and the surrounding structures have almost the 

 same characteristics at both ends of the figure, except that the 

 primitive spinal column is rather more distinct in the posterior end 

 of the section. The posterior cardinal veins (pc), Wolffian ducts 

 (wd), and Wolffian bodies (wt) are also prominent structures of 

 this end of the figure, the last being made up of a great number of 

 tubules. The extreme anterior ends of the Wolffian bodies are seen 

 in the other half of the section in the upper angles of the body 

 cavity, dorsal to the lung rudiments (lu). Filling most of the body 

 cavity (be) and making up the greater part of the middle of the 

 figure are the liver (li), now a very large organ; the stomach (»'), 

 also quite large; the pancreas (pan), a small body lying near the 

 stomach; and the lungs (lu), which here consist of several thick- 

 walled tubes, surrounded by lobes of mesoblast. The other features 

 of the figure need no special mention. 



Figure 20; is through the base of the posterior appendages (pa), 

 in which the cartilages are already being outlined by condensations 

 of mesoblast. The intestine (i) is cut in two regions — at a more 

 anterior point, where it is seen as a small, circular hole surrounded 

 by mesoblast and hung by a narrow mesentery, and through the 

 cloacal region, the larger and more ventral cavity, into which the 

 Wolffian ducts (wd) open a short distance caudad to this section. 

 The blood-vessels present a rather curious appearance. A short 



