Johnston, Forehrain Vesicle in Vertebrates. 



469 



Hoffmann's description (1896) is more complete, but he failed 

 to recognize the source of the mesectoderm beneath the terminal ridge. 

 What he describes as a median mass between the lower border of the 

 neuropore and the "infundibulum" connecting the anterior head 



Fig. 3. Squalus acanthias, 17 somites, median sagittal section. The noto- 

 chord is marked by cross lines and the undifferentiated median mass and pre- 

 oral entoderm by dots. 



nch 



jrch 







k** 



Fig. 4. Detail of a part of the section drawn in Fig 3. x 100- 



cavities with one another is the mesectoderm derived from the termi- 

 nal part of the neural crest to be described below. 



In embryos of 15 somites the notochord is continuous anteriorly 

 with a thickened mass lying over the anterior end of the archen- 

 teron and separated from the entoderm. Laterally the mandibular 



