WP 3 OVA OF MONOTREMES CHAP. 
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. Subse- 
quently Gyldberg” and I* described the ovarian ovum of Hehidna, 
Fic. 46.—Ovarian ege of Hehidna. 6, Basilar membrane; je, follicular epithelium ; 
0, oil globules ; vm, vitelline membrane ; y', 7”, yolk. (Partly after Caldwell. ) 
showing it to be identical with that of Ornithorhynchus. Later 
still a more elaborate and beautifully illustrated paper was pub- 
lished 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. 
' Quart. Journ. Micr. Sci. xxiv. 1884, p. 9. 
2 S.B. Jen. Gesells. 1885, p. 1. 
3 Proc. Roy. Phys. Soc. Edin. viii. 1885, p. 354. 
4 Phil. Trans. clxxviii. 1887. p. 463. 
