1866.] Dr. W. H. Eansom on the Ovum of Osseous Fishes. 227 



Proceedings of the Royal Society in 1854, and of further researches on the 

 structure and properties of the egg in several species of osseous fishes. 

 The methods employed in determining the functions of the micropyle, 

 and in conducting the various inquiries entered upon are described. The 

 development of the ovarian ovum is traced in two species of Gasterosteus, 

 and the yelk-sac is shown to increase by interstitial growth, and not by 

 apposition of layers on either surface. A minute description of the ger- 

 minal vesicle and its contents is given, and the germinal spots are shown to 

 be drops of a thick fluid substance, so apt to change their normally round 

 form, and to vacuolate in their interior, that no perfectly indifferent medium 

 was found in which to examine them. The primitive yelk first formed 

 around the germinal vesicle is shown to differ in some of its chemical and 

 physical properties from that of the ripe ovum ; it is solid, and does not 

 consist of two distinguishable portions. On its surface a yelk-sac was found 

 in very early ova, but in the smallest eggs examined it could not be sepa- 

 rated. 



The reactions of a variety of albumen allied to myosin, which the author 

 has found in variable proportions in the yelk of all the fishes, amphibia, 

 and birds which he has examined, are described, the yelk of the salmon 

 being selected for experiment. This substance, to which the name albumen 

 C. is given provisionally, is remarkable, in addition to its being easily pre- 

 cipitable by water in excess, for forming under certain conditions a solution 

 in dilute nitric acid not coagulable by boiling. 



Some account is rendered of the reactions of an acid compound of phos- 

 phoric acid with an organic substance also met with in the yelk of various 

 animals. 



The phenomena which follow impregnation prior to the commencement 

 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 then 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 con- 

 tracting at the same time to form tke 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, 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 



