GAMETOGENESIS OF ORNITHORHYNCHUS 491 



have begun to form beneath the layer of vacuoles, and the 

 clear space inside will form the body of the latebra. Keference 

 to PI. IB, fig. 5, will show that not all the inner non- vacuolated 

 area (iz) forms latebra, for at ya to yc is an area in which yolk- 

 granules appear in this region. 



In PI. 14, fig. 13, a still later stage is drawn ; this oocyte is 

 interesting because it shows how the formation of coarse yolk- 

 granules (oz) never takes place in the region beneath the 

 nucleus (nu). It is just in this region beneath the nucleus that 

 the cone of protoplasm (cp in Text-fig. 1) and the upper part 

 of the latebra meet to form the so-called Nucleus of Pander. 



The latebra, at a later stage, is shown in PI. 13, fig. 4, d. 

 At UAL the neck of the latebra is distinguishable and the sub- 

 stance of the latebra itself (bl) has become very vacuolated, 

 as indeed has the whole egg, especially after preparation in 

 acetic acid corrosive fixation. 



In another forthcoming paper the appearance of the latebra 

 in the fully-formed egg has been described. It should be 

 noted that the latebra is not formed by the path left by the 

 movement of the nucleus, as is thought by some to be the 

 method of origin of this structure. 



14. A Fully-formed Egg (diameter 4 mm.). 



In PI. 12, fig. 1, is a figure, only slightly diagrammatic, of 

 a fully-formed egg of the duck-billed Platypus. On the 

 outside is the thin shell-membrane (sm), which owing to the 

 contact of the fixatives had become somewhat bent and 

 irregular. Beneath the membrane is a layer of albumen or 

 white (w), which is seen in finished sections as a flocculet 

 lightly-staining substance. The egg-white has been pushed out 

 of place on one side by the bending of the shell-membrane. 



The rest of the egg is formed of the ovum (oocyte) proper. 

 It is bounded on the outside by a very thin membrane (z) 

 called by Caldwell the vitelline membrane, and which I believe 

 to be the zona. In the egg drawn are two distinct areas, an 

 outer (ozy) and an inner (iry) yolk-zone. 



The latebra passes up from the centre of the egg (lzl) to 



