12 J. p. HILL. 



is enclosed by a thin, but very definite refractive membrane 

 or zona (vitelline membrane of Caldwell) of an approximate 

 thickness of '002 mm. (fig. 1, /..p.), on which the cells of the 

 discus proligerus (fig. 1, d.}^.) directly abut, a differentiated 

 corona radiata and syncytial layer being absent. It appears 

 to be identical in its relations and optical characters with the 

 membrane investing the monotreme ovum, and never shows 

 in section any trace of radial striations (though I believe I 

 have detected an extremely faint appearance of such in the 

 fresh zona), or of the extension into it of protoplasmic 

 processes from the adjacent cells of the discus proligerus, 

 such as Caldwell figures in the case of the ovum of Phas- 

 colarctus (cf. his PL 29, fig. 5). Within the zona the 

 peripheral cytoplasm of the ovum is differentiated to form an 

 exceedingly thin but distinct bounding layer or egg-membrane 

 (vitelline membrane, sensu stricto). 



The cytoplasmic body of the ovum exhibits a very obvious 

 and striking differentiation into two regions in correspondence 

 with the presence in it of two definitely localised varieties of 

 deutoplasmic material, resjoectively granular and fluid. Peri- 

 pherally' it consists of a relatively narrow cytoplasmic zone of 

 practicall}'' uniform width, dense and finely granular in 

 appearance owing to the presence in it of numerous particles 

 of deutoplasmic nature. This we may distinguish as the 

 formative zone (fig. 1, f.z.). In it lies embedded the large 

 vesicular nucleus (about '06 x "Oo mm. in diam.). Centrally 

 and forming the main bulk of the ovum is a mass of greatly 

 vacuolated cytoplasm presenting the appearance of a clear 

 wide-meshed reticulum. Its framework is coarser peripherally 

 where it passes over without definite limit into the formative 

 zone, with which it is structurallv identical, but much finer 

 and wider-meshed centrally, so fine, indeed, that it almost 

 invariably breaks down under the action of fixatives, and 

 appears in sections as an irregular space, perhaps crossed by 

 a few fine interlacing strands (fig. 1, d.z.). The meshes of 

 this reticulum are occupied by a clear fluid which must be 

 held to constitute the central deutoplasm of the egg. We 



