GAMETOGENBSIS OF ORxVITHORHYNCHUS 483 



a second vacuolated area appears around the nucleus, known 

 as the perinuclear vacuolated zone. These two zones meet 

 at the animal pole of the egg, above the nucleus, forming the 

 vacuolated nuclear cap. 



Subsequently in this second phase of vitellogenesis the tirst 

 true yolk-spheres put in an appearance firstly in the region of 

 the exoplasm, then more deeply in the endoplasmic region. 

 Van Durme unhesitatingly states that these yolk-spheres 

 partly arise from the larger mitochondria, and partly from the 

 contents of the clear yolk- vesicles (vacuoles). 



From this stage onwards the more deeply-lying mitochondria 

 become fewer, the yolk-elements more numerous, but the 

 cortical mitochondrial zone persists throughout all stages. 



6. The Structure of the Ovary of Ornithorhynchus. 



On taking up a slide of sections of the ovary of Ornithcr- 

 rhynchus and examining it with the naked eye, one is first of 

 all struck by the enormous size of the riper eggs. These are 

 much larger than the full-grown ovarian oocytes of the frog, 

 and of course infinitely larger than those of a rabbit or dog. 

 As in the ovary of a Sauropsidan, the eggs project out around 

 the surface of the organ in a way familiar to any one who has 

 examined the ovary of a fowl or turtle. Thus, while the eggs 

 may be very large, the stroma and general extent of the whole 

 ovary is relatively small. This will be best understood by refer- 

 ence to PI. 12, fig. 3 : in this ovary there was at least one egg 

 nearly if not quite ripe (o), which measured 4-36 mm. in diameter 

 in its shortest way, by 4-52 mm, in its longest way. 



In the ovary drawn in PI. 12, fig. 3, no corpora lutea were 

 to be found, and when these occur they protrude from the 

 surface of the ovary almost as much as the full-grown egg. 

 In several of the ovaries I have examined there are two 

 corpora lutea close together, and these form by far the most 

 prominent structures in the ovaries in question. 



Examined under the low power of a microscope the most 

 striking features of the Ornithorhynchus ovary are the innumer- 



