GAMETOGENESIS OF ORNITHORHYNCHUS 493 



spatulate, but resemble those of reptiles and birds, except that 

 the cytoplasmic part is relativel}'' shorter. 



16. Discussion. 



Probably the most interesting fact ascertained by an examina- 

 tion of this material of the Platypus is the presence of a large 

 hollow cavity in the young ovary. This is undoubtedly 

 a primitive character, which is noticeable even in the adult 

 ovary, in the form of numerous lacunae throughout the stroma 

 of the ovary. 



The stroma of the ovary of the Platypus evidently appears 

 early as a number of separate chords of cells which probably 

 grew into the hollow sac-like ovary at a late stage of embryonic 

 history. The ovary of the original vertebrate seems to have 

 been a sac-like structure, the stroma being a new formation ; 

 the cells which in Ornithorhynchus constitute this loose stroma 

 seem to have been formed by a retro-peritoneal invasion, but 

 as has been pointed out above, they never quite fill the cavity 

 even of the adult ovary. 



The egg of Ornithorhynchus is discharged from the ovary 

 in quite a different way from that of the placental mammal. 

 In the latter the oocyte, with a corona of follicle cells, breaks 

 loose from the glomus proligerus and the release of the egg 

 from its cellular bed involves only part of the follicular elements. 

 In the case of the egg of the Platypus breakage involves the 

 entire follicular layers, as in the case of the frog's ovary, and 

 no liquor folliculi is present or takes part in the expulsion of 

 the egg. 



The formation of the yolk resembles that of the bird described 

 by Van Durme, and the latebra forms in the same manner. 

 Some, at least, of the yolk-spheres are formed as in birds, 

 i. e. by the appearance of watery vacuoles in the ground 

 cytoplasm, and the subsequent loading up of the contents of 

 these vacuoles with fatty and proteid substances, thus con- 

 stituting coagulable and ' soHd ' yolk-spheres. 



The zona appears to be formed from a substance which is 

 intracellular at first ; but it must be admitted that the 



NO. 263 M m 



