200 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



the oil seeins aggregated in the species studied by ine. This oil behaves 

 like oils in general, even in the live eggs, i. e., its specific gravity is less 

 than water, in consequence of which the aggregation of oil drops under- 

 neath the disk tend to keep the latter directed constantly upward. If 

 the egg is turned, the buoyancy of the oil drops at once turns the vitel- 

 lus within the egg membrane, and brings it to rights with the germinal 

 disk looking upwards. This contrivance, if one may so call it, for right- 

 ing the egg is perfectly automatic. Later, when the embryos are hatched, 

 the presence of the oil drops in the upper part of the yelk- sack helps 

 to keep them in the natural position. 



Another totally different type of eggs is that of the cusk, crab-eater, 

 Spanish mackerel, and moon-fish. In all of these forms the egg is buoy- 

 ant in consequence of the presence of a single large oil sphere. This 

 oil sphere is always situated at a point in the egg almost exactly oppo- 

 site the germinal disk. In consequence of this arrangement the ger- 

 minal disk is constantly inverted ; that is, it is carried on the lower- 

 most face of the vitellus, the whole of the latter lying above it. It will 

 be seen that in this case the buoyant oil drop of the egg acts in a man- 

 ner just the reverse of what we noted in the eggs of the salmonoids. 

 Even after the young have escaped from the egg membrane they are at 

 first unable to right themselves, but swim for a time upside down. This 

 is due to the presence of the oil drop in the yelk-sack to the ventral 

 wall of which it is permanently fixed. 



The egg of the cod, strange to say, is wholly without the oil drop, 

 but the specific gravity of the vitellus is so slight that it behaves pre- 

 cisely like the foregoing, and has the germinal disk constantly directed 

 downwards, floating in this position. 



The egg of Morone americana, or white perch, is another special case. 

 Here the egg is adhesive and fixed, and embedded in the vitellus there 

 is a very large oil sphere. In consequence of the fixed character of the 

 egg membrane, the oil-drop controls the position of the vitellus and 

 keeps the disk inverted and on the lower side of the vitelline globe, 

 while the free, uncovered portion of the latter is always directed up- 

 wards, at least during the early stages. 



The egg of the shad is another special case, and here there is an un- 

 usually large water space all around the vitellus between the latter and 

 the egg membrane, but the egg is non-adhesive, and its specific gravity 

 is greater than that of the water in which it is immersed. The pecu- 

 liarity about the behavior of the egg is the constant disposition of the 

 germinal disk to arrange itself at the side of the vitellus when viewed 

 from above, though there is no oil whatever present in the vitellus to 

 influence the position of the vitellus or germ. 



Again, the egg of the plectognath, Aleuteres, is green, with a cluster 

 of oil droplets embedded in its yelk or dentoplasm at one side. Its ger- 

 minal matter or protoplasm is relatively large in amount. 



In Fundulus nud Syngnathus (lie oil drops appear uniformly distrib- 



