DEVELOPMENT OF THE OPOSSUM 17 



nized, the outline of the egg being diffuse and not bounded by a 

 definite line. The zona is 0.004 to 0.0008 mm. in thickness. 



Beneath the zona pellucida is a layer, very light in color and 

 with verj' fine radial striations, clearly seen under the oil-im- 

 mersion lens. In most eggs this has a uniform width of about 

 0.0012 mm. Whether or not this is in any way homologous to 

 the zona radiata I am not prepared to say. 



The cytoplasm is very definitely marked off from the lighter 

 'zona radiata' just mentioned. In fact, in some specimens the 

 line of demarcation is so definite as to amount almost to a 

 membrane. 



As seen in sections the cytoplasm of the egg may be divided 

 into three more or less distinct regions (figs. 1 and 3). The first 

 of these is a narrow homogeneous band of finely granular proto- 

 plasm, devoid of yolk granules or yolk vacuoles. The band is 

 darker than other portions of the cytoplasm, except occasional 

 islands of granular structure surrounding yolk vacuoles. The 

 center of the egg consists of a similar, homogeneous cytoplasm, 

 granular throughout, with only here and there a fat vacuole. 

 The central area extends from the center about half-way to the 

 circumference. Between the marginal and central homogeneous 

 portions of the cytoplasm is a broad band of highly vacuolated, 

 reticular protoplasm, very clearly seen, especially in the eggs of 

 lot No. 76. The structure of this ring seems to be the most 

 delicate of the entire egg, for in two or three specimens the 

 cytoplasm is somewhat broken down at this place. The vacuoles 

 exist, even in eggs fixed with osmic acid mixtures (fig. 1). Sec- 

 tions of such eggs show also a ring of fat spherules, large and 

 small, scattered around the margin of the central yolk free area. 

 The fat spherules are numerous and may occupy a large portion 

 of the vacuolated zone (fig. 1). In the region of the polar body 

 the granular cytoplasm may come to the surface; in other words, 

 there is at this point a break in the vacuolated zone. 



The first polar body and the egg nucleus are to be seen in every 

 specimen sectioned. Neither has undergone any material change 

 since the egg left the ovary. The polar body is very small, 

 containing a mass of chromatin and a minimum of cytoplasm. 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 1 



