AMPHIBIA 229 



cavity is put into communication with the exterior by the 

 enteron posteriorly establishing an opening with the small 

 proctodeum provided by the remnant of the lower lip of the 

 blastopore, and this is speedily converted into the cloaca by 

 the entrance of the pronephric ducts. 



At this stage, therefore, the embryonic enteron is resolved 

 into a wide pharyngeal cavity anteriorly, at the posterior end 

 of which the liver is indicated as a small diverticulum, and a 

 flattened cavity which communicates with the exterior by 

 the cloaca and extends onwards as the postanal enteron to 

 the neurenteric canal. The gills are formed in the pharyngeal 

 region by outgrowths of the endoderm meeting small invagina- 

 tions of the ectoderm, and the mouth by a shallow ingrowth of 

 the ectoderm, the stomodeum, which meets and fuses with 

 the anterior wall of the enteron. 



The notochord is formed from the dorsal endoderm. During 

 the early growth of the dorsal lip two layers only concern 

 this region of the blastopore, the ectoderm above and the 

 endoderm below. The latter is the seat of formation of the 

 notochord. A thickening takes place by multiplication of 

 cells, and the notochord is separated from the roof of the 

 endoderm by simply splitting off from it, and in the process 

 the endoderm may contribute further cellular elements below 

 the notochord, as the so-called hypochorda. The edges of the 

 primitive chorda are continuous with a sheet of mesoderm which 

 is formed by the rest of the lip of the blastopore, and as the 

 notochord separates from the endoderm so does it separate from 

 the rest of the mesoderm. A median notochord is thus formed, 

 and two wings of mesoderm. These are at first directly con- 

 tinuous, and the mesoderm of the ventral lip of the blastopore 

 with the yolk cells of the endoderm. The separation of the 

 notochord from the endoderm is accompanied by the appear- 

 ance of a pair of grooves, one on each side of the notochord, 

 which indicates a contribution of cells from the endoderm in the 

 formation of the myotomes, and is further interesting inrelation 

 to the formation of the myotomes in Amphioxus and in the 

 fish. The mesoderm thus reinforced forms a sheet extending 

 from the notochord, and it gradually meets and fuses under- 

 neath the endoderm. This sheet of mesoderm on each side 



