THE EMBRYOLOGY OF THE OPOSSUM 27 



Soon after the ovum enters the tube albumen begins to be 

 deposited upon the zona in thin concentric sheets, which some- 

 times entrap numerous spermatozoa and occasionally a few 

 epithelial cells from the oviduct. The stratification of the 

 albumen seems to be due to the rolling of the egg during its 

 descent of the oviduct. The final thickness of the albumen 

 before resorption begins is some 0.25 mm., though there is 

 considerable variability in this dimension. 



Just before the egg passes into the uterus a shell membrane 

 is secreted upon the surface of the albumen by the shell glands 

 of the lower part of the oviduct. These glands have been seen 

 by Hartman ('16) and Anderson ('28). The shell is about 

 0.001 mm. in thickness when the ovum reaches the uterus 

 (Hartman, '16). The diameter of the egg including ovum, 

 albumen, and shell is about 0.6 mm. The shell is leathery 

 rather than brittle, and seems, as evidenced by staining re- 

 actions, to be composed of two quite different materials. If 

 not overstained, a clear, yellowish, homogeneous, non-chromo- 

 philic matrix may be distinguished as the principle substance 

 of the shell. Within this, and usually confined to its inner 

 half, is a granular, intensely basophilic, less flexible deposit. 

 When the albumen beneath the shell shrinks during dehydra- 

 tion the shell wrinkles, and the darker material frequently 

 cracks up into many small pieces which remain embedded in 

 the more flexible yellowish material. I mention this detail 

 because I have no doubt that patches of this dark-staining, 

 granular material were what Selenka (1887, plate XVII) mis- 

 took for remnants of the nuclei of granulosa cells. 



Soon after the formation of the second polar body both 

 pronuclei may be seen in the resting stage near the surface of 

 the egg and surrounded by a relatively yolk-free region of 

 cytoplasm. The yolk in other parts of the egg retains its 

 typical sub-peripheral distribution, but the granules and drop- 

 lets have fused into fewer elements which are of correspond- 

 ingly larger size. The two pronuclei now migrate toward the 

 center of the ovum carrying their yolk-free area of cytoplasm 

 with them (fig. 7). As they pass within the limits of the 



