PKOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



EXPLANATION OF PLATE X. 



Kkfeebnces: ab, air-bladder; ao, aorta; Au, auditory vesicle; JS.B, baslbrancMal cartilages; Br, 

 branchial bar; Brs, biancliiosteges; Cb, cerebrum; Cer, cerebeUnm ; ch, chorda; Oh, optio 

 crus ; Ghs, chorda sheath ; Cor, coracoid end of scapular arch ; d, denticles ; df, dental fol- 

 licle; Dm, dermal muscle; ep, epithelium; F, cranial fontanelle; Ghy, glosso-hyal; Hi, 

 hypophysial involution ; Hni, Hyomandibular ; Hy, hypophysis ; H. Hy, Hypo-hyal ; i, 

 intestine; I. H, interbyal; In, infundibulum ; it, iter a tertio ; m, myotomic segments; inb, 

 mid-brain; me, mesoblast ; mcs, mesentery; Mk, Meckcrs cartilage; ms, medulla spinalis; 

 nc, canalis centralis ; "is, cartilaginous or fibro-cartilaginons neural arch ; o, genital folds ; 

 oep, oral epithelium ; Oc, occipital cartilage ; Or, orbit ; Om, ocular muscles ; pp, peritoneal 

 cavity ; pp', i^eritoneal or lymph cavity on either side of the air-bladder ; pi, pigment layer ; 

 Pch, parachordal element; pch, perichondrium; Pv, Pons varolii; Pn, mesonephros; pn, 

 pineal gland ; P. Pt, palato-pterygoid ; Qu, quadrate ; r', r', rays ; rb, cartilaginous axis of 

 ribs ; rb', fibro-cartUaginous investment of libs ; rao, ladix aorta ; s, perichondrial tooth- 

 sockets (cementum plates of pharj"ns) ; Sc, scapular part of shoulder girdle ; S. br, supra- 

 branchial elements ; iSA;, skeletogenous tract from which the vertebra and arches develop ; 

 ssv, supra-spinal vessel; si, deep layer of epidermis; Sy, symplectic ; Tr, trabecula (an- 

 terior part) ; 'T. or, tegmen cranii ; vc, vena cardinales ; Wd, WolfSan ducts. 



Fig. 21. — Chondrocranium of advanced embryo of Gambusia. x 35. 



Fig. 22. — Cross-section through chorda, medulla and incipient vertebraj of the taU. x 65. 



Fig. 23. — Cross-section through an embryo of Gambusia just behind the occiput, x 65. 



Fig. 24. — Cross-section through the tail of a similar embryo, x 365. 



Fig. 25. — Cross-section through the posterior part of the air-bladder and adjacent organs of an embryo 

 of Gambusia. x 96. 



Fig. 26. — Section through a dental follicle and tooth from the pharyngeal region, x 365. 



Fig. 27. — Section through a group of pharyngeal teeth and their supporting plates and branchial bar. 

 x365. 



Fig. 28. — Median section through the brain of an advanced embryo of Gambusia. x 35. 



