CtAmetogrnesir of ornithorhynchur "477 



confined to the edge of the transverse section of the ovary, 

 1. e. on the surface of the ovary ; there does not seem to be any 

 distinct arrangement of follicles, according to size, but the small 

 ones always seem to be near the surface. Poulton noticed that 

 there was evidence that the large follicles were constricted off, 

 in the presence of a deep furrow encircling some of them. 

 By this I believe he means that the egg (and follicle) is con- 

 stricted from outside, and tends to hang somewhat freely on 

 the surface of the ovary. 



Poulton identified a follicular epithelium, which he considered 

 to be of one layer, ' the whole of the time the ovum remains 

 in the follicle '. 



This author also describes faithfully the zona pellucida, 

 follicle, basement membrane, and tunica fibrosa, and establishes 

 the fact that the ' ova of Monotremes practically fill their 

 follicles, and are of considerable size '. The nucleus Poulton 

 considered to be central in the small ova. He recognizes in the 

 older egg a peripheral stainable granular area, and, deeper 

 down, a lighter granular area, beneath which lies the yolk. 



It is remarkable that Poulton should have been able to 

 describe so many interesting facts from such poor material. 



Three years later, in 1887, Caldwell published a paper on 

 ' The Embryology of the Monotremata and Marsupialia ', in 

 which he pointed out that Poulton and Guldberg had wrongly 

 stated that the follicular epithelium remains always a single 

 layer of cells. 



Guldberg and Beddard both described the ovary of Echidna. 

 They showed that it resembled in its oogenesis the condition 

 already described by Poulton for Ornithorhynchus. 



Probably the finest collection of Monotreme material is that 

 procured by Semon about 1893 ; this observer had at his 

 disposal a large number of eggs in all stages. He gives no 

 account of the oogenesis, and his description of the structure 

 of the egg consists of thirty-five lines of general comment, 

 without any detailed account of his material. It is therefore 

 difificult to know how much Semon ipderstood of the structure 

 of the egg. Certain appearances drawn in his figures of the egg 



NO. 263 L 1 



