249 



Fig. A. 



the germinal vesicle has vanished, has an average diameter of about 

 1,33 mm and a volume of 1,2318 cmm. 



3) The egg-capsule becomes thinner, and the pattern on the sur- 

 face stretched out, from the mechanical distension of the ovum. Thus 

 the thickness of the zona of the 

 opaque egg of P. platessa is about 

 0,054 mm; that of the translucent 

 egg 0,025 mm. On the zona of the 

 opaque egg of Brosmius hrosme se- 

 ven or eight punctations lie be- 

 tween two lines 0,05 mm apart, 

 and only two or three in the same 

 space in the mature egg. The 

 thinning of the capsule is however 

 less than calculation shows should 

 occur in the stretching of a homo- 

 geneous elastic membrane to the 

 same extent, a circumstance ac- 

 counted for by its structure. 



Fig. A, B. Sections of the large opaque and of the mature translucent eggs of the 

 plaice (Pleu7-onectes platessa) XiO (slightly diagrammatic), a zona radiata; b, internal 

 vitelline membrane; c, cortical protoplasm. Diameter of ^ 1,21 mm, specific gravity 

 1,070; diameter of È 1,88 mm, specific gravity 1,025. 



4) By the diminished specific gravity. The opaque eggs of all 

 species sink in sea- water, while the mature pelagic float. For example, 



18 



