The Development of the Alhgator 271 



cavities. The former is no longer enclosed, a dor- 

 sal fold in the entoderm being all that remains of 

 the cavity that was seen in the more anterior 

 figures, while the latter is here reduced to a narrow 

 cleft between the somatic and splanchnic meso- 

 blast. A thickening of the mesoblast on either 

 side of the notochord, especially on the left, repre- 

 sents a mesoblastic somite. The medullary canal 

 {mc) is more open than in the more anterior sec- 

 tions. 



For about one third of the length of the embryo 

 posterior to Figure 1 1/ there is a gradual flattening, 

 in a dorso-ventral direction, with loss of the am- 

 nion, until the condition represented in Figure iig 

 is reached. The most striking feature of this region 

 is the great thickness of the ectoderm {ec) , which is 

 still made up of scattered, irregular cells. In the 

 middle line, directly over the medullary canal (here 

 a nearly cylindrical tube) , is a sort of break in the 

 ectoderm, as though there had not been a complete 

 fusion of the epidermal layer when the nervous 

 layer came together on the closure of the medullary 

 groove. This break in the ectoderm may be fol- 

 lowed back to the region of the primitive streak, 

 and will be mentioned again. As has been noted, 

 the medullary canal {mc) is nearly circular in cross- 

 section, and is closely underlaid by the notochord 

 (wi), which is several times the diameter that it 

 was in more anterior sections. The mesoblast 

 ijnes) is a comparatively thin layer, intermediate 



