250 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 diff'erent 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 

 suflficient to distend the yelk-sac, and 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 in a 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. 



