L90 Development of Mouth and Gills in Bdellostoma 
Fics. 1 to 4 illustrating the development of the mandibular arch in 
Bdellostoma x 40 diameters, with Figs. 5 and 6 of this arch in Triton for 
comparison. Fig. 1. Section through the widest part of the mandibular arch 
in a very young embryo. ep, ectodermal mandibular ‘“ gill”’ plates; J, infundi- 
bulum; M, mandibular cleft; Mb, mid-brain; Ms, ventral mesenchyme. Fig. 
2. Widest portion of the arch in slightly older embryo. Fb, fore-brain; Hp, 
hypophysis. Fig. 3. Section of older embryo showing ventral turn of the 
mandibular diverticula. Hb, hind-brain; Dp, dental-plate anlage. Fig. 4. 
Section of a still older embryo showing the wide mandibular cleft in actual 
contact with the ectodermal body wall. Fig. 5. Section through the man- 
dibular arch of Triton, after Greil, compare with Figs. 3 and 4. Lj, lower 
jaw anlage; M, mandibular cleft. Fig. 6. An older Triton embryo after Greil, 
compare Fig. 11 after considering text. Mk, Meckel’s cartilage; n, internal 
nares. 
