488 



J. BRONTE GATENBY 



individual elements of the follicle begins to take place ; the 

 nuclei dividing rapidly, soon become too large and too 

 numerous to lie all in one row in the follicle, and gradually 

 certain nuclei are displaced, as shown in PI. 14, fig. 12, and 

 ultimately a two-layered follicle results (PI. 14, fig. 11, fol). 

 Two-layered the follicle remains all through its subsequent life. 



Now comes one of the events most difficult to understand 

 and interpret — namely, the formation of the zona pellucida. 

 Possibly, however, judging from the accounts of workers who 

 have studied other material, Ornithorhynchus presents the 

 problem in a less difficult form, though there are some points 

 which are still far from clear to me. 



A glance at PI. 14, fig. 9, gives one an impression of the condi- 

 tion of the egg-membrane (om) at this early stage — the mem- 

 brane is a true cell-wall, and nothing else at this period. 



Now in PI. 14, fig. 12, the egg is considerably older, and two 

 new structures have appeared : one is the substance marked 

 pz, the otlier the fibrillae marked cf. The substance marked 

 pz is the precursor of the zona pellucida, while the fibrillae, cf, 

 grow to form the much larger structures shown in PI. 14, fig. 14, 

 at CF. The fibrillae serve as connecting elements between the 

 zona pellucida and the outer cell-membrane (om) of the oocyte 

 cytoplasm. 



In none of the best slides I examined could I be sure that 

 cell-walls existed at the stage draw-nin PI. 14, tig. 12, just when 

 the pre-zona substance is becoming clearly marked. The 

 follicle nuclei appear to lie within a syncytium, but in my 

 mind there exists no doubt that the pre-zona material is formed 

 in or by the follicle cells. The substance might possibly be 

 intercellular, as described by Miss Thing, but it is certainly 

 derived from the follicle ; moreover, up to the last step in the 

 development of the oocyte the follicle cells lie in close relation- 

 ship with the zona, as in PI. 14, fig. 13, and when the egg is 

 extruded the naked edges of the follicle cells are left, apparently 

 supporting the view that the zona and the follicle w^ere pre- 

 viously most intimately related. This is all I can write with 

 reference to the development of the zona. 



