Gastrulation is still more modified and curtailed cenogenetically in the placentals than in the marsupials.
The milk-glands of the polymastic lower placentals are arranged in similar lines.
Placentals do not seem to have existed at the middle of the Mesozoic age--not until towards its close (in the Cretaceous period).
The mammary glands of the Placentals are provided with teats like those of the Marsupials; but we never find in the Placentals the pouch in which the latter carry and suckle their young.
It is only in some of the later Marsupials (Peramelida) and all the Placentals that the allantois develops into the distinctive and remarkable structure that we call the placenta.
Even the important Cenozoic or Tertiary period, in which the first placentals or higher mammals appear, probably amounts to little over two per cent of the whole organic age.
The soul of most of the lower Placentals is not much above that of the reptiles, but among the higher Placentals we find an uninterrupted gradation of mental power up to the apes and man.
The most important of these are the apes and their predecessors, the half-apes, and the earliest Placentals (Prochoriata).
This absence of real bony teeth is a late result of adaptation, as in the toothless Placentals (Edentata, armadillos and ant-eaters).
On the ground of these differences we divide it into two principal sections; the lower Placentals or Indecidua, and the higher Placentals or Deciduata.
Hence the great majority of the Placentals have no direct and close relationship to man, but only the legion of the Primates.
All the higher Placentals that have this decidua are classed together as the "Deciduates.
The non-placentals include only the marsupials and the monotremes (ornithorhyncus and echidna).
There are some two hundred and thirty species of non-placentals in the Australian region.
Of these again, the lowest forms, the monotremes, appear first in the Triassic period; the marsupials in the Jurassic, and then the oldest placentals in the Cretaceous.
Owing to the antiquity of both placentals and marsupials, the arboreal character of the feet of the modern forms of the latter is of little importance.
The monotremes of the Triassic period were followed by the marsupials of the Jurassic, and these by the placentals of the Cretaceous.
Comparative physiology teaches us that the various states of consciousness are just the same in these highest placentals as in man; and we learn by experiment that there is the same reaction to external stimuli.
Man belongs to this, the youngest, sub-class; for he presents in his organization all the features which distinguish the placentals from the marsupials and the still older monotremes.
In the case of the most highly developed placentals this elaborate nutritive arrangement has reached the highest degree of perfection by the construction of a placenta; hence in these classes the embryo is fully developed before birth.
The same formation is present in man and the other placentals in the beginning.
In my latest attempt[9] to arrange the advanced system of placentalsin phylogenetic order I have substituted eight of these legions for the twenty-six orders, and shown that these may be reduced to four main groups.
There are, likewise, other anatomical features, particularly the higher development of the brain and the absence of the marsupial bone, which raise the placentals above all their implacental ancestors.
But the most important of all the recent palaeontological discoveries which have served to elucidate the origin of the placentals relate to our own stem, the legion of primates.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "placentals" in a variety of sentences. We hope that you will now be able to make sentences using this word.
