[117] EMBRYOGRAPHY OF OSSEOUS FISHES. 571 



frees itself from tlie bondage of the vitelline membiane, but it has been 

 known to hatch in thirteen days with the water at 45° Fahr., according 

 to the observations of Mr. R. E. Earll. The time of hatching depends 

 very ranch on the temperature of the sea water, according to the inves- 

 tigator just referred to, where he records the fact that it required as 

 many as fifty days for the eggs to hatch with the water at 31° Fahr., 

 or a little above the freezing point of salt water. Sars says the ova 

 which he had caused to hatch came out in sixteen days. From what 

 we know of the times of hatching of various species, it appears to be 

 the rule that the greatest variation in the time of hatching is found to 

 occur in cases where the spawning takes place with the water at a low 

 temperature; the least variation, on the other hand, appears to occur 

 with those species which spawn when the temperature of the water is 

 comparatively high. An increase of temperature seems to dispro])or- 

 tionately accelerate and abbreviate the rate and time of development, 

 while a decrease appears to disproportionately retard and prolong the 

 rate and time. 



When the young codfish first leaves the egg membrane its tail remains 

 crooked for some time, but soon straightens out, as noted by Sars and 

 Earll. The embryo appears to rupture the egg membrane by spasmodic 

 movements of the tail, which already exerts considerable force as it 

 strikes the membrane, which tends to break open where the head lies, 

 which is most frequently the extremity which first becomes free. This 

 is the fiict, too, with the mackerel and moonfish, but in all of these cases 

 the tail is sometimes the first to become free. The buoyancy of the yelk 

 will for some time tend to keep the young fish turned upon its back, 

 but as soon as the tail has become straight they begin to right them- 

 selves. 



Professor Sars has not recognized the presence of the germinal layer 

 described by me, and his theory of imi)regnation is highly imi^robable, 

 in the face of recent facts. Another jioint remains to be noticed, where 

 lie says, "One can discover, with the aid of a strong niicroscojie, nu 

 merous oil bladders of difierent sizes, and scattered irregularly over 

 the whole surface of the j^elk." He alludes here to what I have denom- 

 inated vesicles, since they are not stained by ha3matoxylon or carmine. 

 As remarked in the introduction, they disappear entirely. It is certain 

 that they coalesce, as he also observes. Are they protoplasmic cor- 

 puscles? If they are, they should stain ; and if fragments of the ger- 

 minative vesicle, they should be still more liable to be tinged by carmine; 

 but lliey do not. Their disappearance and superficial position is j)osi- 

 tive proof that they are not oil spheres. I hold to my original interpre- 

 tation, viz, that they have watery contents. To the Norwegian nat- 

 uralist, however, belongs the credit of having called attention to the 

 fact that certain fish ova fioat and develop at the surface of the water. 



All of the species of floating ova yet investigated by the United States 

 Fish Commission exhibit great mortality in the hatching apparatus, no 



