THE EARLY DEVELOPMEXT OF THE MARSUPIALIA. 33 



superimposed rings, each of eig'lit cells. The upper ring of 

 smaller and clearer cells partially encloses the yolk body, and 

 is situated entirely in the upper hemisphere of the sphere 

 formed by the egg-envelopes. The lower ring of larger, more 

 opaque cells lies approximately in the equatorial region of the 

 said sphere. This 16-celled stage is figured in fig. 55, PL 6, 

 as seen from the side, and in fig. 56 as seen from the upper 

 pole, both figures being taken from a spirit egg "37 mm. in 

 diameter. The marked differences in the cells of the two 

 rings are well brought out in the micro-photographs reproduced 

 as figs. 24, 25, and 20, PI. 2, Figs. 24 and 25 represent 

 horizontal sections of an egg "38 mm. in diameter, the former 

 showing the eight cells of the lower ring, and the latter the 

 eight cells of the upper ring. Fig. 26 shows a vertical section 

 through an egg also of a diameter of "38 mm., but with 

 seventeen cells, one of the original eight cells of the upper ring- 

 having divided and one beingin process of division. The section 

 passes through the yolk-body {y.h.), which is seen as a faintly 

 outlined structure lying in contact with the zona between the 

 two cells of the upper ring (/.c). 



The shell-membrane in eggs of this 16-celled stage has 

 attained a thickness of "0075 mm., and the albumen layer has 

 been almost cotnpletely absorbed, so that the zona now lies 

 practically in apposition with the shell-membrane, the two 

 together forming a firm resistant sphere, to the inner surface 

 of which the blastomeres are closely applied. The separation 

 between the zona and shell-membrane seen in the figures is 

 largely, if not wholly, artificial. 



The average measurements of the cells of the two rings in 

 the "38 mm. egg, figui-ed in figs. 24 and 25, are as follows : 

 Upper ring cells. Lower ring cells. 



Diameter . "06 x -058 mm. . -09 x '064 mm. 



Vertical height '095 mm. . -115 mm. 



Nucleus . . •0165 mm. . •02 mm. 



These measurements demonstrate at a glance the distinct 

 difference in size which exists between the cells of the two 

 rings, whilst the cytoplasmic differences between them are 



VOL. 56, PART 1. — NEW SERIES. 3 



