72 W. B. Kirkham — Maturation of the Egg of the White Mouse. 



The Mature Ovicm. — The e^^ of tlie mouse is as small as any 

 known mammalian egg. A living ovarian ovum,, before the forma- 

 tion of the first maturation spindle, measures .08"'"\, or a little less 

 in diameter, and it contains a germinal vesicle with a diameter of 

 .025"'"\ while this germinal vesicle may in turn possess one or two 

 nucleoli with diameters of .OOS'""". Living eggs in the Fallopian 

 tube, before cleavage, measure from .073 to 078""™ in diameter. 

 Sobotta's statement that the egg of the mouse measures .059™'" in 

 diameter is probably based upon his study of sections after the eggs 

 had become shrunken through the action of the reagents used. The 

 human egg, for comparison, is .165 to .170""" in diameter, that of 

 the cat measures .2"'™, or a little less, while the egg of the rabbit 

 is .116™"* in diameter. 



Figure 1. — Camera drawing of a living egg soon after its discharge into the 

 Fallopian tube ; showing the zona pellucida made up of an inner, thinner, 

 and dense layer, and an outer, thicker, gelatinous layer. The cytoplasm of 

 the egg is coarsely gi'anular except at one spot, which indicates the position 

 of the second polar spindle. This egg has not retained the first polar body. 

 x880. 



The zona pellucida. surrounding a tube Qgg in the mouse usually 

 has a thickness of about .016"'™, but it may stretch out to .036""". 

 It is made up of a denser inner layer, with a thickness of about 

 .007™™, which is all ihat appears around ovarian eggs, and an outer 

 layer of less dense but perfectly homogeneous substance (Text-figs. 

 1-3). After being killed and dehydrated, the zona appears ver}- 

 much shrunken (PI. YIII, fig. 16), which probably accounts for 

 Sobotta's statement that the Qgg of the mouse is envelo|)ed in the 

 thinnest known mammalian zona pellucida, which has a thickness of 

 .0019 to .0015™™. In the mouse this membrane lacks the radical 

 striations or canals which have given it the name of zona radiata. 



