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 discovery rests with Owen and with Professor Poulton, who pointed out in 1884, that the ovum of Ornithorhynchus is very large as compared with those of other Mammalia (6 mm. as against .2 mm.), that it is filled with yolk, and that it completely fills the follicle, being surrounded by two layers of follicular cells only. This latter fact was proved by Caldwell. Subsequently Gyldberg and I described the ovarian ovum of Echidna, showing it to be identical with that of Ornithorhynchus. Later still a more elaborate and beautifully illustrated paper was published by Caldwell upon the early stages of development in the Monotremata and Marsupials, in which the ovum of the former was accurately described (see Fig. 46). In the particulars mentioned above, the ovum of the Monotremata is practically identical with that of the large-yolked ova of the Sauropsida.