The Development of the AlHgator 261 



the preceding section, and passes through the point 

 of separation of the folds of the entoderm (en). 

 From this point the entoderm gradually flattens 

 out, leaving the enteron unenclosed. The medul- 

 lary canal (mc) and notochord (nt) are about as 

 in the preceding section, but the ectoderm (ep) 

 is somewhat thinner and more flattened. The 

 mesoderm {mes) on the right side exhibits a distinct 

 cleavage, the resulting body cavity (be) being a 

 large, triangular space. 



Figure gg, the twenty-fifth section posterior 

 to that represented in Figure 9/, shows a marked 

 change in the form of the embryo. While of 

 about the same lateral dimensions, the dorso- 

 ventral diameter of the embryo in this region is 

 less than one half what it was in the head region. 

 The epidermal ectoderm (ep) is now nearly hori- 

 zontal in position and is not so abruptly separated 

 laterally from the thin lateral sheets of ectoblast. 

 The medullary groove (mg) is here a very narrow 

 vertical slit. At this stage the fusion of the medul- 

 lary folds has taken place over the anterior third 

 of the embryo. For a short distance, represented 

 in about thirty-five sections, the canal is open 

 as in the figure under discussion; for the next one 

 hundred sections (about one third the length of 

 the embryo) in the region of the mesoblastic somites 

 the canal is again closed, while throughout the last 

 one third of its length the canal is widely open dor- 

 sally. The enteron is here entirely open ventrally, 



