494 Development of Mouth and Gills in Bdellostoma 
Fias. 7 to 14. Sections showing the development of the dental-plate or 
lower jaw. X 41 diameters. Fig. 7. Through the anterior part of the mouth 
showing the dental-plate anlage in a young embryo. cp, the thin median 
body; lh, two large anterior prongs, all of these parts are united further back. 
Fig. 8. A more posterior section of the same embryo, the median body, cp, 
is here united with the anterior prongs, /h. The anterior prong on the right 
side is seen united also with the ventral wall of the head. M, mouth cavity. 
Fig. 9. Section of the anterior portion of the dental-plate in older embryo, /h, 
the anterior prongs have grown ahead of the median body in their forward 
extention. Hp, hypophysis; 7, tentacles. Fig. 10. Section through the same 
embryo in a more posterior region than Fig. 9. The anterior prongs, /h, are 
here joined to the median body, cp. The entire dental-plate anlage has its 
ventral wall in common with the ventral wall of the head just as has the lower 
jaw of the Triton embryo in Fig. 5. Fig. 11. Section through the anterior 
prongs, /h, of the dental-plate showing the teeth anlage on their median faces. 
ht, teeth; Bv, blood-vessels; C, mandibular cartilage; 7, section of tentacle; 
T’, section through base of more posterior tentacle. Fig. 12. A more posterior 
section than Fig. 11, the two forward prongs of the dental-plate are joined 
together medio-ventrally, and laterally to the head. Their ventral wall is 
the ventral wall of the head as would be expected of the lower jaw. Tc, 
throat cavity; x, a place where the forward prong is still free from the head 
not far enough posterior to give the complete union. Fig. 13. Through the 
longitudinal mouth of an old embryo. /f, Fleshy lip folds the anterior lateral 
extentions of the dental-plate body; Bs, blood sinus. Fig. 14. A more pos- 
terior section through the same embryo. Shows dental-plate with teeth, ht, 
developing on its dorsal surface; (C, basal cartilage of dental-plate; Te, 
throat. 
