THE PLAOENTATION OF PERAMELES. 



445 



Alimentary 

 canal, &c. 



P. nasuta, Stage C. 



7 mm. 

 from crown to rump. 



Heart and 



vessels 



Urino- 

 genital 

 system 



Q^lsophagusopen. Se- 

 parate dorsal and 

 ventral pancreas an- 

 lagen. Cloaca not 

 open to exterior. 

 Lateral tliymus an- 

 lagen. Yolk - sac 

 cavity opens into 

 gut. 



P. obesula, Stage D. 



8-75 mm. 

 from crown to rump. 



P. nasuta, new-born. 



Stajie E. 



14 mm. crown to rump. 



Head length 6 mm. 



Pancreas completely 

 formed. Tliymus 

 aniagea approxima- 

 ted in their mid- 

 regions. Lungs with 

 numerous simple 

 alveoli. Cartilagin- 

 ous rings round tra- 

 chea. Diaj)hragm 

 complete. 



Distinct septum supe- 

 rius and septum 

 spurium. Com- 

 mencing division of 

 the ventricular ca- 

 vity indicated by an 

 internal fold and 

 external groove. 

 Transversely ex- 

 panded sinus veno- 

 sus, opening into 

 right auricular divi- 

 sion. Truncusaortse 

 undivided. Two an- 

 terior dorsal aortae. 

 Yolk - sac circula- 

 tion predominant. 



Mesonephros of con- 

 siderable size, tu- 

 bules convoluted 

 and with distinct 

 glomeruli. Wolf- 

 fian ducts open into 

 cloaca togetherwith 

 allantoic canal. Dis- 

 tinct supra - renal 

 anlagen. Genital- 

 leisten small. 



Cloaca just opened to 

 exterior, still re- 

 mains of cloaca! 

 membrane. Yolk- 

 sac no longer opens 

 into gut. Thymus 

 anlagen united me 

 sially, but free pos 

 teriorly ; they now 

 reach the pericar- 

 dium. Lungs now 

 lobed, the right 

 with a ventro-mesial lobe. Bronchi have 

 branched out to form secondary bronchi. 

 Diaphragm still incomplete close to the 

 mesial line. 



Ventricular septum 

 (septum inferius) 

 almost reaches the 

 cushions of the au- 

 riculo - ventricular 

 ostium, likewise the 

 septum superius, 

 the auricles only 

 communicating be- 

 low its concave 

 edge. Sinus veno- 

 sus less extensive. 

 Inferior vena cava 

 established. Allan- 

 toic circulation pre- 

 dominant. 



Mesonephros now of 

 large size. Peri- 

 toneal funnels of the 

 Mullerian ducts laid 

 down. Anlagen ol 

 ureters asshortout- 

 growths from the 

 posterior ends of 

 the Wolffian ducts, 

 surrounded by con- 

 densed mesenchyme. 

 Genital-leisten dis- 

 tinct, indifferent (?). 



Adult circulation. Po- 

 sition of allantoic 

 arteries still recog- 

 nisable in urachus. 



Mesonephros still of 

 great size. Muller- 

 ian ducts laid down 

 for part of their 

 course. Permanent 

 kidney definitely es- 

 tablished and of 

 some size, with tu- 

 buli contorti. Ure- 

 tersopenintocloaca 

 mesially to Wolffian 

 ducts. Projecting 

 supra-renal anlagen. 

 Prominent genital- 

 leisten, (^ (?). 



