48() J. BRONTE GATENBY 



Ornithorhynclius. The one complete egg- and shell-membrane 

 of which I examined sections was only from 4-5 to 5 mm. 

 in diameter, though it was difficult owing to the wrinkling to 

 make an accurate measurement (PI. 12, fig. 1), 



9. The Young Oocyte of the Platypus. 



Sonu' oocytes which had just undergone the prophases of 

 the h(!terotypic division were discovered in the Plemming-fixed 

 material ; two such oocytes are drawn on PI. 14, figs. 7 and 8. 

 The nucleus is nearly always spherical, but occasionally 

 irregular as shown in fig. 8 ; there is a well-marked nucleolus, 

 NU in fig. 7, of the fragmented type ; in some nuclei the nucleolus 

 can be seen to be formed of two parts — a lightly-staining region, 

 NOP in fig. 10, and a darkly-staining region, nub. In fig. 11 

 the nucleolus consists of a very large darkly-staining sphere 

 and a number of smaller pale elements ; the chromatin is 

 feebly staining and dispersed in all these nuclei. 



In nearly all the young oocytes observed a ceutrosphere 

 is present, cs in figs. 7 and 8 ; the centrosphere at this stage 

 lies near the nucleus, often within a dent in the nuclear mem- 

 brane, as in PI. 14, fig. 8. In some cases contrioles or small 

 granules within the centrosphere can be made out, as in 

 fig. 8, cs. In the youngest oocytes the centrosphere may be 

 surrounded by a cloud of granules which have been identified 

 as mitochondria (m), fig. 7. 



In older oocytes the mitochondria, as happens in all verte- 

 brate eggs, gradually pass away from the ceutrosphere, and 

 become spread out into the cytoplasm (fig. 8) ; they tend to 

 collect as matted granules and filaments, particularly in the 

 region of the periphery of the egg, and become difficult to 

 demonstrate at and after this period. 



10. On the Early Establishment of a Polarity in 

 the Platypus Oocyte. 



All the oocytes examined showed a distinct polarity, in 

 that the nucleus had taken up a position to one side of the 

 oocyte cytoplasm. I believe that this polarity has no relation- 



