486 CHORDATA. 
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. 
4. 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 Me¢atheria, all the 
higher forms being known as Eutheria. 
