502 Development of Mouth and Gills in Bdellostoma 
Fias. 15 to 18 illustrate the degeneration of the hyomandibular cleft x 44 
diameters. Fig. 15. Through the widest portion of the cleft in a very young 
embryo. On the left side the endodermal and ectodermal thickenings are 
almost in contact. ep, ectodermal hyomandibular gill pocket; ent, endo- 
dermal thickening; Ch, chorda; Hb, hind-brain. Fig. 16. Section of an older 
embryo showing increased space between ectodermal and endodermal parts of 
the gill. Fig. 17. Section of still older one. The ectodermal thickenings 
have disappeared and the endodermal diverticulum, Hm, is less extensive; 
v, anlage of velum. Fig. 18. Section of old embryo, the slight diverticulum, 
Hm, is all that remains of the hyomandibular cleft. A, ear; Ca, cartilage of 
the auditory box. 
Frias. 19 to 22. Sections showing the degeneration of the first post- 
hyomandibular gill. Lettered as.above. Fig. 19. Section of a very young 
embryo. Fig. 20. Older embryo showing the gill further developed. 2¢t, the 
anterior beginning of the second post-hyomandibular gill. Fig. 21. Section 
of older embryo showing degeneration of the ectodermal thickening, and the 
endodermal diverticulum is less extensive at this age. p, pharynx. Fig. 22. 
Embryo in which this gill is almost lost, there being no ectodermal indication 
of it, and the throat diverticulum is very small. 
Fics. 23 to 25 show sections of the first branchial gill in early stages of 
its development. Fig. 23. From the same young embryo as Fig. 19. Fig. 24. 
From the same embryo as Fig. 20; np, branchial nerve placode. Fig. 25. 
Older embryo with the endodermal diverticulum, ent, much increased in 
extent; g2, g3, anterior edges of the diverticula of the 2d and 3d branchial 
gills. Fig. 26. Guide to indicate the embryonic areas from which the above 
sections were taken. Hm, region of the hyomandibular ones. J, the first 
post-hyomandibular ones. Br, the first branchial gill region. 
