72 OVA OF MONOTREMES chap. 



discovery rests with Owen and with Professor Poulton, who 

 pointed out in 1884/ that the ovum of Ornithorhyncltus 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 l)y two layers of follicular 

 cells only. This latter fact was proved by Caldwell. Subse- 

 quently Gyldberg ^ and I ^ descriljed the ovarian ovum of Ecltidna, 



- — 3H'!'7^?^^.- 







..sf'af- • 



'■<A #' * V 







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

 (), oil globules : vid, vitelline membrane ; //^ ,'/'". yolk. (Partly after Caldwell.) 



showing it to be identical with that of OrnWiorhyiichiis. Later 

 still a more elaborate and beautifully illustrated paper was puV)- 

 lished by Caldwell'^ upon the early stages of development in the 

 Monotremata and IVIarsupials, 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 w^ith that of the large-yolked ova of the Sauropsida. 



^ Quart. Journ. MLcr. Sci. xxiv. 1884, p. 9. 

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

 ■* Proc. Boy. Phys. Soc. Edin. viii. 188.5, p. 354. 

 •* Phil. Trans, clxxviii. 1887- p. 463. 



