3) (a Royal Society :— 
phosphoric acid with an organic substance also met with in the yelk 
of various animals. 
The phenomena which follow impregnation prior to the com- 
mencement of cleavage are described, and are shown to be chiefly 
due to the influence upon the yelk of water which has passed through 
the yelk-sac. 
Some variations which occur in this respect in different species of 
osseous fishes are described; and the ova of Gasterosteus are shown 
to be remarkable in having a viscid mucoid covering derived from 
the oviduct, which prevents the imbibition of water through the 
yelk-sac, so that it only enters and forms a breathing-chamber 
after impregnation, when it passes through the aperture in the apex 
of the micropyle; whereas in the eggs of salmon and in those of 
most other fishes, unimpregnated ova “rapidly absorb water by the 
whole surface of the yelk-sac, the yelk contracting at the same time 
to form the breathing-chamber. 
The concentration of the formative yelk, originally forming a thin 
layer over the whole yelk-ball, at the germinal pole is also proved 
to be due to the action of water, of which it requires a free supply 
sufficient to distend the yelk-sac, snl to be independent of fecundation. 
The contractions of the yelk are shown to be also independent of 
the action of the spermatozoids, and to be reactions following the 
entrance of water into the breathing-chamber—and this not only as 
regards the rhythmic waves which pass over the surface of the food- 
yelk, but also the fissile contractility of the formative yelk, by virtue 
of which it cleaves into irregular and unsymmetrical masses, and 
which the author conceives to be only regulated by the influence of 
the seminal particles. 
The cortical layer of the food-yelk or inner sac, which is shown to 
resist ina remarkable manner osmosis, is found to be the rhythmically 
contractile part, although requiring for its manifestation the presence 
of acid food-yelk upon its inner surface. 
Evidence is given to show that the contractile property of the yelk 
of both kinds requires, as an essential condition of its manifestation, 
the presence of oxygen in the surrounding medium, and that the 
food-yelk, while the rhythmic waves are passing over it, consumes 
less than does the formative yelk, while regularly cleaving after 
fecundation,—also that some product of oxidation is formed during 
these movements, which itself tends to check them, but which the 
author failed to determine the nature of. 
Proofs are also given that a certain moderate rise of temperature 
increases the activity of these contractions. Experiments are related 
which show the extreme limits the yelk will bear without destroying 
them, and the temperature at which commencing chemical change 
prevents further contraction. 
_ The reactions of the substance of the yelk under the stimulus of 
galvanism are recorded, and evidence afforded that the food-yelk 
and the cortical layer alone are excited to contraction by it, attempts 
made to induce fissile or other contractions of the formative yelk 
resulting in electrolysis of that highly unstable substance. 
