492 J. BRONTE OATKNBY 



the region generally called the Nucleus of Pander (ual) beneath 

 the hlastodisk (bu), in which the nucleus (nu) is situated. 



For further details of the egg proper the other paper should be 

 consulted. The neck of the latebra and the region of the Nucleus 

 of Pander has been described therein more fully. 



Faithful drawings of the shell-membrane and its under- 

 lying areas have been made by Caldwell, and will be found in 

 his paper. 



The average size of the fully-formed egg of the platypus is 

 about 4 mm. ; but, as Wilson and Hill have pointed out, it 

 soon absorbs liquid from the uterine walls, and grows to 12 or 

 14 mm. at the time of laying. 



15. Note on Spermatogenesis. 



Among Professor J. P. Hill's material were some sections 

 of Ornithorhynchus testis, and in PI. 13, fig. 6, I have given 

 a drawing of a part of one semeniferous tubule and some 

 interstitial tissue. Very good figures of the spermatozoa have 

 previously been made by Retzius and Benda. 



Two of the most striking facts about the histology of the 

 platypus testis are the large size of the cells of the interstitial 

 tissue (int) and the remarkable development of the Sertoli 

 cells (sc). The latter seem to be derived directly from the 

 basement cells or primitive spermatogonia, and I could find 

 nothing suggestive of the presence of any kind of Sertoli cell 

 determinant as described by Montgomery for man. In the 

 platypus the primitive spermatogonium probably becomes 

 a Sertoli cell merely in sympathy with the development of 

 a group of spermatocytes above it. At spt (lower) are a group 

 of spermatocytes nearly full grown, and beneath them, at ysc, 

 is a cell which is in the same series as the primitive spermato- 

 gonia above and below (spg), but which is hypertrophying step 

 by step with the group of spermatocytes near by. At so is 

 a Sertoli cell ready for the fixation of the spermatids (spd^) 

 which is just beginning, and at spd" is a Sertoli cell with the 

 later spermatids all attached. At spz is a group of ripe sperms 

 attached to a fullv-formed Sertoli cell. The sperms are not 



