EMBRYOLOGY OF THE MARSUPIALIA. 15 



Ear. — Semicircular canals formed. Cochlea bent. Periotic 

 capsule partly pro- cartilage, partly cartilage. External audi- 

 tory meatus plugged by epidermal cells. 



Nose. — Much as in new-born. Choanae formed; turbinal 

 projections arising, and Jacobson's organ formed and its 

 cartilage present. Solid lachrymal ducts, which reach the 

 nasal epithelium. 



Mouth. — Palate formed, and invests glottis posteriorly. 

 Club-like tooth germs, but no enamel organs. Tongue 

 grooved and with distinct taste-buds. Submaxillary salivary 

 glands open into mouth. 



Alimentary Canal, etc. — Lateral thyroid Anlagen on 

 sides of trachea. Thymus Anlagen approximated in their 

 middle regions, but free posteriorly. Extensive pancreas, 

 its duct opening into bile-duct. Lungs with numerous simple 

 alveoli. Cartilaginous rings round trachea. Diaphi-agm 

 complete and muscle-fibres developing in it. Intestinal loop 

 no longer projects at navel. 



Heart and Vessels. — Ventricular septum not yet com- 

 plete. Auricular septum complete and a foramen ovale 

 present. Sinus venosus still extensive. Truncus aortse 

 almost completely divided. Left ductus Botalli present. 

 Single dorsal aorta. Renal portal circulation in connection 

 with Wolffian bodies. Allantoic circulation greatly pre- 

 dominates over yolk-sac circulation. 



Urogenital System. — Mesonephros of large size. Peri- 

 toneal funnels of Mullerian ducts present. Commencing 

 renal tubules. Ureters open into urogenital sinus mesially 

 to Wolffian ducts. Genital Leisten prominent. Sex ? 



Skin and Skeleton. — Thin nucleated epitrichial layer 

 on epidermis. Hair Anlagen well advanced on head. No ossi- 

 fication in cartilaginous skeleton. Pre-maxillary, maxillary, 

 and palatine ossifications. Rudimentary clavicle ossified.^ 



1 This is also present in the 8"75 mm. embryo of Stage D. 



