26 J. p. HILL. 



characterises the ripe ovai'ian ovum (the upper pole beiug 

 marked by the vacuolated deiitoplasinic zone (ligs. 8-11, cl.z.), 

 and so far as its cytophismic body is concerned it shows no 

 essential difference from that. 



Examined fresh in normal salt solution, the formative cyto- 

 plasm forming the bulk of the ovum appears dense, finely 

 granular, and of a very faint lightish-brown tint, its opacit}'" 

 being such that the two pronuclei situated in its central region 

 can just be made out. In section, this central region is dis- 

 tinguishable from the peripheral zone by its uniform, more 

 finely granular character and by the absence of the fluid-filled 

 vacuolar spaces which are generally present in the latter 

 figs. 10 and 12). The deutoplasmic zone at the upper pole, 

 which is only partially visible in the entire egg owing to the way 

 in which it is enclosed by the formative cytoplasm (figs. 8, 9, 

 d.y..), presents a characteristically clear or semi-transparent 

 vacuolated appearance in the fresh state, but may have em- 

 bedded in it a small dense mass (fig. 8, cf. also figs. 11 and 14), 

 evidently formed by the transformation of a portion of its fluid 

 constitutent into the solid state, and so to be regarded as com- 

 parable with a bit of formative cytoplasm. 



In most of the unsegmeiited uterine ova at my disposal the 

 male and female pronuclei have attained approximately the 

 same size aud lie in proximity in the central more homo- 

 geneous region of the formative cytoplasm (figs. 10-12). The 

 transformation of the sperm-head into the male pronucleus 

 probably takes place during the passage of the ovum down 

 the tube, and was not observed, aud I am as yet uncertain 

 Avhether the pronuclei unite to form a single cleavage nucleus 

 or give origin directly to the chromosomes of the first cleavage 

 figure. 



Caldwell figures ('87, PI. oO, fig. 5) a section through the 

 uterine ovum of Phascolarctus which I reproduce here as 

 Text-fig, l,iu order to facilitate comparison with my figs. 11 and 

 12, with which it shows an essential agreement, apart from 

 the presence of follicular cells in the albumen which I have 

 never observed in Dasyurus, and making allowance for the 



