GAMETOGKNEFUS OF ORNITHORHYNCHUS 489 



In a well-advanced oocyte the zona and the underlying 

 structures appear as drawn in PI. 14, fig, 14. The zona has 

 stained black with haematoxylin ; beneath the zona is the 

 true cell-wall of the egg (om), which is quite thick. I call this 

 the true cell-wall of the egg because I believe it can be traced 

 back to the undoubted cell-wall of the earliest oocyte, marked 

 OM, in PI. 14, fig. 9. In PI. 14, fig. 14, the cell-wall (om) is con- 

 nected to the zona by a large number of cortical fibrillae ; 

 these, marked cf in PL 14, figs. 12 and 14, probably serve the 

 dual purpose of attaching the zona firmly to the egg, and of 

 acting as living protoplasmic connexions between the nutrient 

 bringing follicle and the receptive interior of the egg. 



Outside the theca itself is possibly another layer of less 

 closely packed, often obscurely defined cells, which can be 

 recognized as a theca externa, distinguishable from the true 

 theca, or theca interna (PL 14, fig. 13, th and ostr). The theca 

 externa, like the true or inner theca, is formed by cells which, 

 sympathetic to the development of oocyte, become slightly 

 flattened and help to form a supporting and vascular capsule 

 for the egg. 



12. Yolk Formation in the Platypus. 



The egg of Ornithorhynchus is macrolecithal and an extremely 

 difficult object to section. Its yolk, like that of the frog's egg, 

 stains densely in iron alum haematoxylin. PL 12, fig. 3, gives 

 an idea of the appearance of a section stained by this method. 

 In another paper ^ on the full-grown egg (shortly in press), 

 in PL 1, fig. 1, the enormous number of yolk-granules can be 

 noted. After fixation of the ovary in acetic acid fixatives, 

 the formation of the yolk is seen apparently to be heralded by 

 the appearance of a number of vacuoles beneath the periphery 

 of the egg. These vacuoles, which are shown in PL 13, fig. 4, 

 at B, are probably filled with a lipoid substance of some sort, 

 for at this stage the egg preserved in chrome-osmium does not 

 exhibit such vacuoles. 



^ In this paper is described the polar body formation and minute struc- 

 ture of the latebra of the maturing ovum. 



