458 
TK. W. H. EANSOM ON THE OVUM OF OSSEOUS FISHES. 
can determine the prompt entry of the surrounding medium, unless it is assumed that 
the inward current is assisted by a contraction of that part of the protoplasm with which 
the spermatozooid comes into contact*. I tried in vain to observe the entry with the 
stream into the egg, through the micropyle, of minute particles of carmine while the 
breathing-chamber was forming. A solution of caramel somewhat acid decomposes 
the viscid layer, and stains the yelk-sac, but cannot be seen to colour the fluid in the 
breathing-chamber ; and the same result was obtained by using a salt of iron tested 
afterwards with prussiate of potash. It is, however, difficult to tell the colour of the 
contents of the breathing-chamber seen through a dyed yelk-sac. A watery solution of 
the extract of safflower gave a similar result, and after twenty minutes, on rupture all 
the contents of the egg were colourless ; the inner sac was especially noted to be colour- 
less. It would seem that the viscid layer does not entirely prevent the absorption of 
watery solutions of pigments into the substance of the yelk-sac. This does not, however, 
invalidate the conclusion as to the mode in which water enters to form the breathing- 
chamber. That it is the inner sac which presents the obstacle to the imbibition of 
aqueous solutions of colouring-matter into the yelk, is shown by the fact that eggs which 
have lain for forty-eight hours in the dead body of the parent, and have become slightly 
decomposed, permit the tint of the caramel, and of iron when tested by prussiate of 
potash, to appear in the substance of the yelk, in such of them only as have the inner 
sac ruptured, a change which often occurs in dead eggs, and will be again referred to. 
b. Concentration of the formative yelk . — Very soon after the funnel of the micropyle 
begins to shorten, the formative yelk commences to undergo the series of changes which 
eventually terminate in the formation of the germinal disk. 
In one instance, where the spermatozooid was seen to enter, the yellow droplets were 
distinctly paler in minute, and an obscure puckering was visible at the same time 
on the surface of the discus proligerus, which, after the completion of the act of fecun- 
dation, I would call the germinal disk. Gradually all the granular and other elements 
of the cortical layer or formative yelk move away from the ventral segment, and con- 
centrate into a disk at the germinal pole, where it then covers a somewhat smaller area 
than that previously occupied by the discus proligerus, but is thicker. The egg becomes 
clearer in consequence, partly through the removal from the surface of the clear food- 
yelk of a granular opaque layer, partly from the distension and increased translucency of 
the yelk-sac. At the same time the structural elements of the cortical layer undergo 
certain changes, which show that some slight action of water takes place through the 
substance of the inner sac. The yellow droplets grow paler, and disappear without 
distinct vacuolation, commencing to pale first at the germinal pole. Although, in a 
short interval of time after these changes begin, all, or nearly all the granules of the 
cortex are transferred to the germinal pole, and the yellow droplets either carried with 
them or in some way rendered invisible, I failed to see any distinct movement of them 
streaming towards the germinal pole. 
* Such contractions have been shown by Newport to take place in the eggs of frogs. 
