486 



Causes and Course of Organic Evolution 



Batrachia (Urodela) 



(11) Vertebral segments having 

 cartilage formed from connective 

 tissue, also cartilaginous interverte- 

 brse, which in time fuse with verte- 

 brae. 



(12) Axis vertebra has ribs. 



(13) Ribs are double-headed. 



(14) Epipubic cartilages in most. 

 Carpal and tarsal bones of nine 

 pieces in higher genera and that 

 are usually distinct. 



(15) Cerebellum fully exposed, and 

 corpus callosum rudimentary. 



(16) Cloaca well developed. 



(17) Lungs long and simple to 

 condensed and complex, lobulate. 



(18) An allantoic placenta as a 

 saccular outgrowth of ventral cloacal 

 wall that functions as urinary bladder. 



(19) Eggs holoblastic approaching 

 to meroblastic in segmentation, with 

 unequal cleavage. 



Mammalia (excepting 

 Monotremata) 



(11) Vertebral segments with car- 

 tilage formed directly in the ver- 

 tebra^, later intervertebral masses 

 remain as distinct bony epiphyses. 



(12) Axis ribs in Perameles and 

 Phascolo^jale, but absorbed in other 

 mammals higher. 



(13) Ribs are double-headed (single 

 in Protothei ia) . 



(14) Epipubic cartilages in Thjf- 

 lacinus, but becoming bones in other 

 marsupials, possibly also in dog. 

 Carpal and tarsal bones of nine 

 pieces that are distinct or may some- 

 what fuse with each other. 



(15) Cerebellum fully exposed, and 

 corpus callosum rudimentary in mar- 

 supials and lower mammals. 



(16) Cloaca rudimentary in mar- 

 supials only. 



(17) Lungs rarely long and simple, 

 usually complex and lobulate. 



(18) Allantoic placenta present in 

 most higher mammals, and in mar- 

 supial Perameles; absent in other 

 marsupials. 



(19) Eggs holoblastic rarely ap- 

 proaching to meroblastic (Marsupi- 

 alia), with equal segmentation. 



But in addition to the above characters the pelvic arch 

 forms a feature of contact between the two groups that was 

 strongly and deservedly emphasized by Huxley. For, apart 

 from the epipubic cartilages or bones of higher urodeles and 

 their homology (Duges, Cuvier, Huxley) with the marsupial 

 bones of mammals, the relative develoj^ment and jiosition 

 of the ilium, ischium, and pubis in Salamandra as compared 

 with marsupials, caused Huxley to say (76'0.-404): "These 

 facts appear to me to point to the conclusion that the Mam- 

 malia have been connected with the Amphibia by some un- 

 known promammalian group, and not by any of the known 

 forms of Sauropsida." 



