The Development of the AHigator 281 



about forty sections, or one fifth of the length of the 

 embryo. Their exact length is difficult to deter- 

 mine 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 anteriorly 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, passing, in fact, on the left side 

 through the extreme edge of its lateral fold, which 

 is shown as a upward bend in the ectoblast and 

 somatopleure. 



The ectoblast (ec) shows the same remarkable 

 thickening that was noted in the corresponding 

 region of the preceding stage. The spinal cord 

 (sc), notochord {nt), aortae (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 aortae, 

 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. 



