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J- BIIONTE GATENBY 



able lacunae or spaces in the ground-work or stroma. The 

 biggest of these spaces are drawn in PI. 12, fig. 3, being cross- 

 hatched (ca), but to gain a better understanding of this pecu- 

 liarity one must examine tig. 4 of PI. 13. Here the extraordinary 

 structure of the ovary is demonstrated, a well-marked germinal 

 epithelium is recognizable (ge), and beneath it are a row of 

 oocytes in various stages ; on the right of PI. 13, fig. 4, the 

 oocytes are found to lie in a more solid cortical area of the ovary, 

 which is marked off at this region quite sharply by the wide and 

 numerous lacunae, with their trabeculae in between (tr). 

 These cavities do not contain blood, or lymph corpuscles, but 

 seem to have been occupietl by a non-corpuscular fluid, which 

 leaves no trace of coaguluni in the finished sections. 



As the young oocytes grow older they tend to become com- 

 pletely surrounded by strands of nmch vacuolated tissue, as is 

 nidicated in tlic largest oocyte drawn in PI. 18, fig. 4. This 

 feature is certainly one of the most remarkable in the ovary 

 of Ornithorhynchus. It will therefore be clear that by the 

 time an egg has reached the stage drawn in PI. 13, fig. 4 (roughly 

 one-eighth of its full size), it is already floating in a basket-like 

 area formed by connective-tissue trabeculae and intervening 

 lacunae tilled with liquid. 



7. The Appearance of the Immature Ovary of the 



Platypus. 



In PI. 12, fig. 2, is drawn an immature ovary measuring 

 3-250xl() mm. This shows remarkably well^he almost 

 a,mphibian character of the ovary at this stage. As was pointed 

 out above with reference to the mature ovary, there is also to 

 be seen in this innnature specimen a cortical arrangement of 

 oocytes ; around the ovary the eggs tend to lie in a thickened 

 area, beneath which is a space occupying the centre of the 

 organ. This cavity is only partly filled with loose strands of 

 connective tissue. 



One is forced to look upon this peculiar structure of the imma- 

 ture ovary of Ornithorhynchus as a very primitive feature. 



