GAMETOGENESIS OF ORNITHORHYNCHUS 487 



ship to the plane of the surface of the ovary, nuclei being found 

 lying inwards, outwards, or sideways to an axis drawn directly 

 down at right angles to the surface of the gonad. 



From the material examined it is impossible to understand 

 completely the mode of origin of the polarity in the young 

 oocytes, but from our knowledge of many vertebrate oogonia 

 we are aware that when in this early stage the nucleus tends to 

 lie to one side of the cell. The polarity of the Ornithorhynchus 

 oocyte is therefore probably established during the oogonial 

 stage, either as the accidental result of the position of the centro- 

 somes and centrospheres of the daughter-cells during oogonial 

 divisions, or as a subsequent and more expressly determined 

 movement of the oogonial nucleus within the cytoplasm, 

 at a stage just before the inception of the prophases of the 

 heterotypic divisions. The former is most likely. 



This polarity of the oocytes persists throughout their entire 

 growth, marking permanently the position of blastoderm and 

 vegetative pole of the full-grown oocyte, and of the part of the 

 egg in which the latebra will be formed. 



11. Formation of Egg-membranes. 



The egg-membranes on the ovarian oocyte of Ornitho- 

 rhynchus are a theca (externa and interna), a follicle, and 

 a zona pellucida. 



In all the youngest oocytes that have been observed the 

 follicle is well formed ; it is shown in PI. 14, figs. 7 and 8, fol, 

 and much enlarged in fig. 9. In the latter figure the follicle 

 is seen to consist of one layer of flattened cells, overlying the 

 substances of the oocyte (oc). In good preparations it is 

 possible to recognize clearly a limiting or true cell-membrane 

 around the egg-cytoplasm, om, in PL 14, fig. 9. Distinct cell- 

 walls between the individual cell elements of the follicle were 

 generally difficult to find, but are probably always present. 



In PI. 14, fig. 12, the same region of an older oocyte is drawn. 

 The follicle cells as such could not be identified in this prepara- 

 tion, but the nuclei and general cell-substance have increased 

 greatly in size. Just at this stage a new arrangement of the 



