[25] EMBRYOGRAPHY OF OSSEOUS FISHES. 479 



wriukle, becomes gradually thinner, and finally disappears altogether; 

 the already numerous nucleoli become still more so, but also smaller 

 and smaller, until they are finally indistinguishable, so that one is 

 obliged to conclude that they have been dissolved in the intranuclear 

 fluid. Finally, the nucleus is brought to lie close against the zona ra- 

 diata, as a wall-less, irregular, viscous, almost homogeneous mass; the 

 intranuclear fluid in which the nucleoli have been dissolved now begins 

 to mix with the egg contents. Accompanying this blending important 

 changes occur, and, as a result of the process, the direction spindle, the 

 nucleus, and the yelk are difierentiated. In the pellucid ova of Scor- 

 pmia the yelk spherules are again broken down and the food-yelk 

 then forms a clear, semi-fluid mass ; in JuUs, Serrmius^ and Fierasfer, 

 the clear yelk also contains a large, shining oil-sphere; in Grcnilahrus 

 the yelk is not entirely pellucid, but contains some, not very numerous, 

 yelk granules ; in the herring and Heliasis the yelk contains a great 

 number of yelk spherules, but which, in consequence of their larger 

 size and less bright appearance, look very djfterently from the yelk 

 spherules of the immature ^gg. 



"The direction spindle [polar spindle j has its peripheral pole placed 

 immediately against the inner opening of the micropyle. It is best seen 

 in Scorpwna, in which it has a length of .025"^"', and a diameter of 

 .0145™™; its longitudinal axis forms an angle of 45° with that of the 

 axis of the egg. It is less easily made out in JuUs, while the eggs of 

 Crenilabrm, Heliasis, Gohitis, Blennius, Belone, Clupea, are too unsatis- 

 factory and opaque as objects in which to seek for the direction si)in- 

 dle. 



"The form of the germ in the mature ova taken from the females of 

 different species, without being brought into contact with water, is very 

 different. In Julis it surrounds, as a relatively thick layer, the entire 

 yelk, and is thickest at the micropylar pole of the egg; in Scotpcena 

 it covers in a cap-like manner the micropylar pole of the yelk where it 

 is thickest, and becomes gradually thinner towards the equator, where 

 it disappears almost wholly, but is continued over the opposite pole as 

 a distinct but very thin layer. In the eggs of the herring and Heliasis 

 it does not form a very thick stratum beneath the micropyle, and extends 

 from this region as irregular thick and thin processes down amongst the 

 yelk spheres of the whole egg. The direction spindle also is always 

 imbedded in the germ disk. Kupfler's statement that the germ of the 

 herring is developed nnder the influence of the sea-water an<l the sperm 

 rests upon inaccurate observations; in the unim])regnated egg of the 

 herring, as in Heliasis, the germinal matter still remains in great part 

 strewn amongst the yelk spherules, as is apparently the case with all 

 eggs in which the yelk does not (consist of fluid material, but for the most 

 part of larger and snuiUcr yelk spherules. 



"The fact that the yelk spherules of pelagic eggs should again be 

 broken down and in mature eggs become clear and pellucid throughout 

 must probably be regarded as a phenomenon of adaptation, the whole 



