BY PROFESSOR J. T. WILSON. 735 



lUict of Jacobson's organ. The neuro-epithelium of the latter is well shown 



(J.o.e.). 



Plate xli. 



Fig. 20 represents a coronal section at about the plane of the posterior 

 margin of the lamella marked s.p.c. in fig. 3. It passes through the inter- 

 nasal aperture {i.a.) below the septum nasi, and shows the now tapering 

 posterior ends of Jacobson's cartilages. 



Fig. 21 is immediately behind the plane of the internasal aperture; the tip 

 of Jacobson's cartilage on one side is still present, whilst the vomer has 

 appeared as a splint on the ventral border of the cartilaginous septum nasi. 

 Instead of the cartilaginous lamella (s.p.c.) seen in fig. 20, we now have, as 

 skeletal basis of the secondary palate, the palatine plates of the m^axillse 

 {mx'.). Note here, as well as on one side of the previous figure, the compact 

 bundle of nerve-fibres from the olfactory bulb passing towards the hinder 

 extremity of Jacobson's organ to supply its neuro-epithelium. 



Plate xlii. 



Fig. 22. Portion of coronal section of snout of specimen "A" of foetal 

 Ornithorhynchus. Section No. 23, of series. Hematoxylin and picric acid 

 staining. Magnification of 100 diameters. The most dorsal portion of the os 

 earunculas exhibits an area of apparently cartilaginous character. Its cells 

 are larger, vesicular, and have very distinct capsules around them. 



General list of reference letters, 



cd.n., alinasal; al.n'., free anterior margin of alinasal; al.n.f., alinasal 

 forming nasal floor; al.n.t., alinasal turbinal ; ant.nar., anterior nares; 

 cap., cartilage of narial aperture; cap. a., cartilage of narial aperture, anterior 

 moiety; cap.p., cartilage of narial aperture, posterior moiety; fen., fenestra 

 of cartilaginous septum nasi; gl., glandular tissue; i.a., internasal aperture 

 above secondary palate; J.c, cartilaginous capsule of Jacobson's organ; 

 f.J.c , foramen in cartilaginous capsule of Jacobson's organ; J.o.e. , epithelium 

 of Jacobson's organ; J.ol., olfactory nerve-bundle for Jacobson's organ; 

 m.c, marginal cartilage of snout, expansion from nasal floor cartilage; 

 r.m.c, rostral continuation of above by side of prerostral notch; mx. , maxilla; 

 mx'., palatine plate of maxilla; n., nasal bone; n.n., nasal nerve; n.d., nasal 

 duct; ?i./., nasal floor; ii.??., narial valves; ?i.2?.c., naso-palatine canal; o.c.,os 

 carunculte ; pr.n., prerostral notch; p.p.c, cartilage of papilla palatina; 

 p.vc, prevomer (dumb-bell ossification); px., ventral lamella of body of 

 premaxilla; px'., trabecula connecting os carunculiB with rest of premaxilla; 

 px"., dorsal lamella of body of premaxilla; p.px., palatine process of pre- 

 maxilla; s.n., septum nasi; s.n'., septum nasi where it descends into the 



