32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



quite unenclosed. The canal of the spinal cord (sc) is smaller in 

 proportion to the thickness of its walls, and the notochord (rii) is 

 somewhat larger than in the preceding sections. In proportion to 

 its extent, the ectoderm is very thick. Under the notochord the 

 dorsal aortse (ao) are seen as two large, round openings in the 

 mesoblast. On the left side the section passes through the center 

 of a somite and shows a small, round myocoel (myc). The meso- 

 blastic layer of the amnion (so^ is distinct throughout from the ecto- 

 blastic layer (a). 



The most important structures to be here noted are the first rudi- 

 ments of the Wolffian ducts (wd). They are seen in the present 

 section as lateral ridges of mesoblast projecting outward and up- 

 ward toward the ectoblast, which suddenly becomes thin as it passes 

 over them. These ridges or cords of mesoblast are as yet quite 

 solid. They arise suddenly at about the eightieth section of the 

 series of two hundred and may be traced through about forty sec- 

 tions, or one-fifth of the length of the embryo. Their exact length 

 is difficult to determine because, while their anterior ends are blunt 

 and sharply defined, they taper so gradually posteriorly that it is 

 hard to tell just where they end. They apparently originate ante- 

 riorly and gradually extend toward the tail. In a slightly younger 

 embryo the rudimentary Wolffian duct could be seen as a still 

 smaller rod of cells extending posteriorly for a few sections, from 

 the seventy-fifth section of a series of about two hundred. In the 

 particular series under discussion the left rudimentary Wolffian duct 

 was about one-fifth longer than the right one. 



Figure 12/ is just posterior to the head-fold of the amnion, pass- 

 ing, in fact, on the left side through the extreme edge of its lateral 

 fold, which is shown as an upward bend in the ectoblast and somat- 

 opleure. 



The ectoblast (cc) shows the same remarkable thickening that 

 was noted in the corresponding region of the preceding stage. The 

 spinal cord (sc), notochord (nt), aorta? (ao), and entoderm (en) 

 need no special mention. The mesoderm seems to be separated by 

 unusually wide spaces from both ectoderm and entoderm, and is 

 made up of rather closely packed cells except around the aorta?, 

 where there seems scarcely enough tissue to hold these vessels in 

 place. The body cavity (be) is large, and a small myocoel (myc) 

 is seen on the left. 



Figure I2g is through the neurenteric canal (11c), a distinct open- 

 ing through the floor of the spinal canal. The section is of course 

 just back of the posterior end of the notochord. The entoderm 

 (en) along the margin of the neurenteric canal is naturally contin- 



