110 OUTLINES OF CHORDATE DEVELOPMENT 



form a wedge-shaped elevation continuous with the endoderm; 

 this is the rudiment of the notochord. In a still later stage 

 the chorda begins to be cut off from the endoderm by a narrow 

 split leaving the enteron roofed dorsally by a layer only one 

 cell thick (Fig. 44). Passing posteriorly from the first section 

 described above, into the region of the blastopore, we find the 

 grooves out of the archenteron better marked and the ventral 

 lip, as well as the dorsal, quite pronounced. These grooves 

 are apparently indications of the enterocoelic evaginations; 



ec 



FIG. 36. Part of a transverse section through the young embryo of R. fusca, 

 showing traces of enteroccel formation. After O. Hertwig. a, Archenteron; 

 c, enteroccels; ec, ectoderm; en, endoderm; m, mesoderm; n, notochord; p, neural 

 plate; y, yolk cells. 



this relation will be mentioned more fully at the close of the 

 description of notogenesis. 



7. The Medullary Plate 



The rudiment of another axial structure is developing at the 

 same time as the chorda and mesoderm; this is the medullary 

 plate. The medullary plate is formed in part from the median 

 band of cells extending from the region of the dorsal lip of the 

 blastopore nearly to the animal pole, and in part from the 

 axial thickening due to the confluence of the lateral portions 

 of the germ ring. In the former region the inner or nervous 

 layer of ectoderm begins to thicken and by the time the blasto- 

 pore has become circular and commenced to close, a thickened 

 medullary plate has formed over the whole dorsal surface of 

 the gastrula (Fig. 32, F). This is in the form of a broad plate, 



