251 



51,5 ce of very pale yellow almost transparent fluid, which was found 

 by Dr. Milroy to possess a specific gravity of 1,0116, a practically 

 neutral reaction, and to contain a large amount of chlorides, phos- 

 phates being absent; the amount of chloride of sodium present per 

 100 ccm was 0,766 grammes, the total albumins being 0,838 grammes; 

 no mucin or pseudomucin was detected. It may be stated that opaque 

 eggs placed in weak solution of common salt clear up considerably, so 

 as to show the germinal vesicle, which persists. 



It thus appears that the final stage in the development 

 of the ovarian eggs of Tele osteans is characterised by the 

 comparatively rapid imbibition of a watery, and probably 

 saline, fluid of low density, which seems to be secreted by 

 the follicle, and is associated with the dissolution of the 

 germinal vesicle and the rearrangement of the chromatin. 

 In eggs which become pelagic the volume of fluid imbibed 

 is relativel y very large, dissolving also the yolk-spheru_ 

 les and reducing the specific gravity of the whole ovum 

 below that of sea-water. 



It is in virtue of this change that the eggs of certain marine species 

 become buoyant or pelagic (those of fresh-water species could not by 

 this device be enabled to float) ; but in as much as it also occurs in 

 demersal eggs it is clear that its signification lies deeper. It is pro- 

 bable that not only among Teleosteans but among some 

 other groups the apparent vanishing of the germinal 

 vesicle and the rearrangement of chromatin is due to 

 the absorption of fluid from without, and that this is the 

 primary and essential meaning of the process. In certain 

 marine Teleosteans Natural Selection has, so to speak, seized upon 

 and exaggerated a normal phenomenon for another purpose, viz. to 

 enable the eggs to float and be dispersed. The same genus may com- 

 prise closely-allied species whose eggs are respectively pelagic and 

 demersal. 



The fact that the yolk of pelagic eggs contain so large a proportion 

 of water explains some other phenomena, such as the situation of the 

 heavier blastodisc at the lower pole, the more rapid and less perfect 

 embryonic development, the absence of vitelline circulation and 

 the simple manner in which the yolk is absorbed. The initial nutri- 

 tive value of a unit of yolk is less in a pelagic than in a demersal 

 egg, owing to the less condensed^ or more fluid, character of the 

 spherules ; but the nutritive value is further greatly diminished 

 by the dilution of the yolk with three or four times its volume of 

 water. For example, the demersal egg of Cottus scorpius is of about 



18* 



