1887.] Embryology of Monotremata and Marsupialia. Ill 



x 2 ), and as we have 

 r = ihVB. 



But li\/3 is a diameter of the hexagonal prism perpendicular to two 

 of its opposite faces ; hence a sphere may be inscribed within the 

 cell from a point measured from the vertex at a distance equal to the 

 side of one of the lozenges, and with a radius equal to half the long 

 diagonal of this lozenge, while another sphere with a diameter equal 

 to three times the side of the lozenge circumscribes the triangular 

 pyramid at the summit. 



The diameter D' of the inscribed sphere is equal to the diameter of 

 the cell, while the diameter D of the exterior sphere is equal to the 

 sum of three edges of the pyramid, and it therefore follows from the 

 first note, vol. 41, that between these diameters we have the 

 expression — 



d _ /?\| 



d' - \2j ■ 



The relation between the geometrical cell and the interior and 

 exterior spheres whose diameters are connected by this equation may 

 possibly have some bearing on the question of the formation of the 

 •actual cells. 



perpendicular is manifestly equal to v — 

 •evidently — 



r 6 



we obtain 



III. " The Embryology of Monotremata and Marsupialia. Part I." 

 By W. H. Caldwell, M.A., Fellow of Gonville and Caius 

 College, Cambridge. Communicated by Prof. M. Foster, 

 Sec. R.S. Eeceived February 22, 1887. 



(Abstract.)* 



1. The B 'gg -membranes. 



In Monotremata, in very young ova, a fine membrane exists between 

 the single row of follicular cells and the substance of the ovum. This 

 membrane, which I will call the vitelline membrane, at first increases 

 in thickness with the growth of the ovum, and through it pass nume- 

 rous fine protoplasmic processes connecting the protoplasm of the 

 follicular cells with that of the ovum, and serving to conduct food 

 granules, which, appearing in the neighbourhood of the nuclei of the 



* The author being at the present time in Australia and so unable to correct the 

 proof of this abstract, I have undertaken this duty. In doing so I have ventured, 

 for the sake of what appeared to be increased clearness, to introduce into § 1 some 

 modifications of tbe author's manuscript, being guided therein by the author's more 

 detailed account given in the fuller paper. — M. Fostee, Sec. K.S. 



VOL. XLII. O 



