Originally in the reptilian ancestor, or in the monotreme, the ovum in the follicle secreted yellow-coloured yolk.
"Hormones and Heredity"
J. T. Cunningham
It is more difficult to identify monotreme remains, but the fact that monotremes have survived to this day in Australia, and the resemblance of some of the Mesozoic teeth to those found for a time in the young Duckbill justify us in assuming that a part of the Mesozoic mammals correspond to the modern monotremes.
"The Story of Evolution"
Joseph McCabe
It is enough that the living monotreme and Marsupial so finely illustrate the transition from a reptilian to a mammalian form.
"The Story of Evolution"
Joseph McCabe