OVA OF MONOTREMES 



discovery rests with Owen and with Professor Poulton, who 

 pointed out in 1884, 1 that the ovum of Ornithorliynchus 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 2 and 1 3 described the ovarian ovum of Echidna, 



FIG. 46. Ovarian egg of Echidna, b, Basilar membrane ; fe, follicular epithelium ; 

 o, oil globules ; vm, vitelline membrane ; y l , y 2 , yolk. (Partly after Caldwell.) 



showing it to be identical with that of Ornithorhynclius. Later 

 still a more elaborate and beautifully illustrated paper was pub- 

 lished by Caldwell 4 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. 



1 Quart. Journ. Micr. Sei. xxiv. 1884, p. 9. 



- S.B. Jen. Gesells. 1885, p. 1. 



3 Proc. Roy. Phy's. Soc. Edin. viii. 1885, p. 354. 



4 Phil. Trans, clxxviii. 1887. p. 463. 



