486 CHORD AT A. 



3. Closure of blastopore and entry of ovum into uterus, 

 accompanied by rapid increase of embryo to form blasto- 

 cyst. 



4. Division of epiblast into embryonic disc and extra- 

 embryonic part, which afterwards forms amnion and serosa ; 

 and growth of hypoblast round inside of serosa, the disc 

 forming embryonic hypoblast, the vesicular wall the yolk-sac. 



5. Attachment of serosa by villi to the uterine wall. 



6. Addition of mesoblastic covering to yolk-sac, growth 

 of allantois and growth of yolk-sac villi to form yolk-sac 

 placenta. 



7. Growth of allantoic villi into the uterine tissues and 

 attendant changes, producing the true allantoic placenta. 

 Atrophy of yolk-sac. 



8. Birth of embryo by rupture of serosa and amnion, 

 followed by shedding of after-birth or placenta. Termination 

 of uterine gestation. 



9. Commencement of mammary gestation. 



Classification' of Mammalia. 



Mammalia have, as we believe, been descended from 

 amphibio-reptiles in the past, so those mammals which still 

 present us with reptilian characters must take the lowest 

 place. Of these we find that two small mammals, the duck- 

 mole and the porcupine anteater, differ from all other 

 mammals in having an oviparous habit, so we are con- 

 strained to emphasise this fact by putting them into a sub- 

 order by themselves, called Protofheria (first quadrupeds) or 

 Ornithodelphia. This distinction is further corroborated by 

 numerous anatomical characters. The extant sub-class Pro- 

 totheria have but one order, the Monotremata. All the 

 other mammals are viviparous, but almost the whole of the 

 indigenous mammals of Australia and a few allies in America 

 show a simpler condition of the reproductive organs and 

 along with this a much less pronounced viviparous habit. 

 The young are born at a very early stage and there is. in all 

 but a single exception, no true allantoic placenta. These 

 and other features enable us to divide the *' marsupial '' 

 animals from the rest into the sub-class Metatheria, all the 

 higher forms being known as Eutheria. 



