GAMETOGENESIS OF ORNITHORHYNCHUS 495 



14. Bonnet, R. — ' Lehrbuch der Entwicklungsgeschichte ', Vicrte Aufl.', 



1920. 



15. Gatenby and Woodger. — " On the Relationship between the Forma- 



tion of Yolk and the INIitochondria and Golgi Apparatus durmg 

 Oogenesis ", ' Journ. Roy. Micr. Soc.', 1920. 



Explanation of Lettering. 



A, youngest oocyte found, at, area of attachment of ovary to body- wall. 

 B, oocyte at beginning of formation of latebra. bd, blastoderm, bl, inner 

 region of egg, which will form part of latebra. bv, blood-vessel, c, egg at 

 stage of beginning of yolk formation. CA, cavities in ovary. CF, cortical 

 fibrillae, beneath zona, en, chromatin, cs, centrosphere. dy, dark- 

 staining yolk. FOL, follicle of egg. ge, germinal epithelium, int, inter- 

 stitial tissue of testis, try, inner region of yolk, ivz, inner vacuolated 

 zone of egg. iz, innermost zone of oocyte, lzl, lower zone of latebra. 

 M, mitochondria. NU, nucleus, nub, darkly-staining nucleolus. Nup, 

 faintlj'-stainhig nucleolus, oc, cytoplasm of oocyte, ofol, outer limiting 

 membrane of follicle, om, cell-wall of oocyte, oo, oocyte, oox, small 

 oocyte compressed by growth of a larger one. ostr, outer region of theca 

 (theca externa), oz, outer or peripheral zone of oocyte. OZY, outer zone 

 of yolk. PY, pale yolk. PZ, pre-zona, or substance which forms zona 

 pellucida. sc, Sertoli cell of testis, sm, shell-membrane, spd, 1 and 2, 

 two stages of spermatids, spg, spermatogonium, spt, spermatocyte. 

 SPZ, spermatozoon, th, theca (uaterna). tr, ovarian trabeculae. ual, 

 upper area of latebra. v, vacuole, w, egg white, x, material formed 

 probably by degeneration of oocytes, xy, enigmatic plasmatic body in 

 young oocytes, ya, yb, yc, stages in formation of yolk-spheres, z, zona. 



DESCEIPTION OF PLATES. 

 Plate 12. 



Fig. 1. — Fully-formed egg of Ornithorhynchus paradoxus, 

 in vertical section. Shows latebra, yolk, albumen, and shell- membrane. 



Fig. 2. — Transverse section of a young ovary showing cavity (ca) and 

 loose trabeculae (tr), and cortically arranged oocytes (oo). 



Fig. 3.^ — Fully-developed ovary of Ornithorhynchus, oocytes blacked in. 

 Cavities in stroma (ca) cross-hatched. 



Plate 13. 



Fig. 4. — Part of adult ovary more highly magnified showing oocytes 

 in different stages. The numerous cavities in the stroma are evident, and 

 several stages in the formation of the latebra are given (b, c, d). In the 

 egg D, the follicle is only put in below. 



Fig. 5. — Part of the egg at an early stage of yolk formation, as in 

 fig. 4, c. Cortical vacuoles and deeper yolk formations are shown. 



M m 2 



