72 OVA OF MOXOTREMES chap. 



discovery rests with Owen and with Professor Poulton, who 

 pointed out in 1884/ that the ovum of OndthorlbyTiclius 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 G-yldberg ^ and I ^ described the ovarian ovum of Echidna, 



<'t 







I ^4 i 







» '»' 



Fig. 46.— Ovarian egg of Echidna, h, Ba.silar membrane; fe, follicular epithelium; 

 0, oil globules ; vni, vitelline membrane ; y^, y-, yolk. (Partly after Caldwell. ) 



showing it to Ije identical with that of Ornithorhynclms. Later 

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

 lished by Caldwell * upon the early stages of development in the 

 .AI(jnotremata 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 with that of the large-yolked ova of the Sauropsida. 



' Quart. Joiirii. Micr. Sci. xxix. 1884, p. 9. 

 " !?.B. Jen. Gesells. 1885, p. 1. 

 ■' Prm:. Roy. Phys. Soc. Min. viii. 1885, p. 354. 

 '' Phil. Trans, olxxviii. 1887. p. 463. 



