559 



aorta. The cells of the anterior part of this vessel, as well as those 

 of the aortic vessels of the mandibular arch, arise from the wall of 

 to closed endodermic tube. 



Thus the median floor of the alimentary canal first fuses with 

 the ectoderm, then touches the anterior surface of the ventral 

 mesoderm , then forms the anterior wall of the ventral aorta , and 

 finally turns horizontally as the ventral floor of the branchial 

 chamber. 



o/<. 



post. 



md mes ih c of 



Fier. 3. 



vid + hy 



meet 



th mes 



Fig. 5. 



hb cl ■' 



Fig. 1. Outline of oral fusion. (In the figures, the nuclei are represented by 

 circles and the yolk granules by solid dots.) 



Figs. 2, 3, 4, 5. Cross-sections through an embryo about 8 mm in length. The 

 figures are limited above by the hyobranchial cleft. Fig. 2 is the anterior section, and 

 three sections intervene between the successive sections figured, h c body cavity, hh cl 

 hyobranchial cleft. hm cl hyomandibular cleft, hy mes hyoid mesoderm, ma ao man- 

 dibular aortic arch, mdmes mandibular mesoderm, meet mesectoderm. mes mesoderm. 

 of oral fusion, th thyroid outgrowth, th c thyroid cavity, v a ventral aorta. 



