154 OUTLINES OF CHORDATE DEVELOPMENT 



FIG. 52. The development of the olfactory organ in R. fusca. After Hins- 

 berg. A, B, C. Sections through the olfactory groove and organ of 5 mm., 6 mm., 

 and 11 mm. larvae, respectively. D. Lateral view of a model of the olfactory 

 organ of a 31 mm. larva. The dotted line marks the limit between the sensory 

 and non-sensory portions of the epithelial lining of the olfactory cavities, c, 

 Notochord; ch, internal nares (choanae); d, dorsal lumen; dc, dorsal sac; en, ex- 

 ternal nares; g, olfactory groove; i, cut edge of integument; in, internal nares 

 (choanse) ; I, elongation toward the mouth; la, lateral appendix; m, mouth cavity; 

 n, inner or nervous layer of ectoderm; ns, part of chamber lined with non-sensory 

 epithelium; p, olfactory placode; r, ridge marking the limit between middle and 

 ventral chambers; s, superficial layer of ectoderm; se, part of chamber lined with 

 sensory epithelium; st, stomodseum; t, telencephalon; v, thickened bands of super- 

 ficial ectoderm cells (possibly the vestige of a primitive sense organ) ; re, ventral 

 sac; vg, ventral nasal gland attached to Jacobson's organ; x, elevation around 

 external nares; y, canal leading to olfactory cavity; z, fold around internal narial 

 opening. 



