OVUM IN THE CAPE AND NEW ZEALAND PEEIPATUS. 293 



4. The nucleus becomes vacuolate. 



5. The vacuolate tissue of the nucleus passes to its border 

 and there forms a thick wall ; the centre of the nucleus in 

 which the nucleolus lies is clear and does not stain. 



6. The wall of the nucleus loses its vacuolate structure and 

 becomes solid and homogeneous. 



7. During these changes in the nucleus the ovum has 

 increased in size, the shell has become thick, and small round, 

 highly-refractive yolk- granules have arisen in the protoplasm 

 of the cell. 



8. The nucleus acquires an irregular outline, which is caused 

 by peripheral portions of its substance passing out into the 

 cell plasma, where they are visible as spheres of various sizes. 



9. The whole of the nucleus with the exception of the 

 nucleolus breaks up in this way ; the spheres to which it gives 

 rise probably become metamorphosed into yolk. 



10. The protoplasm becomes vacuolate, the boundary between 

 the egg and its follicular stalk disappears, and nuclei pass from 

 the latter into the substance of the former. The nucleolus is 

 not visible. 



11. The follicular stalk becomes hollow, and nuclei from its 

 walls, and yolk pass along it into the ovum from the ovary. 



12. The follicular nuclei are no longer visible and probably 

 become transformed into yolk. 



13. A large nucleus appears in the ovum, lying near its 

 point of attachment to the stalk, the boundary between the 

 two being again established. The origin of this nucleus is not 

 known. 



14. The youngest ovum in the uterus has no nucleus visible. 



15. The origin of the segmentation nucleus is unknown. 



16. No polar bodies have been observed. 



General Considerations. 



Recently much work has been done on the subject of the 

 origin of the ova and the phenomena of maturation and ferti- 

 lization. I shall not attempt here to give a complete review 



