BULLETIN OF THE UNITED STATES FISH COMMISSION. 201 



uted and embedded in the superficial portions of the yelk next to its 

 external surface. This brings the deposits of oily matter in close prox- 

 imity to the vessels traversing the vitellus. The function of these oils, 

 aside from their buoyant tendencies, as in these last cases, is not clear, 

 and, beyond the fact that they are evidently absorbed together with the 

 remainder of the yelk, we know little of their nutritive properties. Per- 

 haps, in the process of physiological decomposition, these oils of fish 

 embryos develop heat. If we are to judge by what may be observed in 

 the absorption of the single oil drop of the Spanish mackerel, for ex- 

 ample, it is one of the last portions of the vitellus to be absorbed. In 

 fact, it dwindles progressively, the drop continually growing smaller, 

 while it retains its globular form as it disappears. 



The remarkable differences here noted in regard to the organization 

 and behavior of fish eggs are noteworthy, too, as showing that the state- 

 ment so often and unwarrantably made, even by very distinguished 

 biologists, to the effect that ova in general have the same physical con- 

 stitution, differ in no respect from each other, &c, has no foundation in 

 fact. Indeed, the more intimately we know the various forms of ova of 

 the various animal species, the more evident does it become that some 

 time, when our knowledge is more complete, we shall perhaps be able 

 to distinguish the species apart by the eggs alone, just as botanists have 

 used the characters presented by seeds to distinguish plants. It is also 

 evident that such striking diversities of organization must, to a certain 

 extent, be reflected in the mode of development of the various species ; 

 that independently of the action of the principle of acceleration and re- 

 tardation, pointed out by Professor Cope (the effects of the working of 

 which are frequently very evident), the morphological character of the 

 egg reacts upon the manner of development, as proved by the one fact 

 that with the variation in the bulk of the yelk there is a corresponding 

 variation in the length of the arc embraced by the body of the embyro 

 as it lies on the sac before the tail begins to bud out. When once the 

 body is segmented, and the primary somites are distinguishable, as we 

 may call those proper to the body, which are formed before the caudal 

 somites, the amount of matter used up in carrying the development to 

 this stage in different species will of course vary j the yelk itself also 

 varies in dimensions ; hence, as a natural result, the arc on the great 

 longitudinal circumference of the latter, embraced by the embyro, must 

 in like manner vary, so as to comprise one-third, one-half, or three- 

 fourths of that circumference, as may be noted in different forms. These 

 are not the only consequences of structure, as we learn upon making a 

 still wider survey of the various forms of fish ova, some of which have 

 been alluded to elsewhere, though we may here note the fact that the 

 number of primary somites varies in the embryos of different genera. 

 So great is this difference at the same relative stage in different spe- 

 cies, that we find as many as seventy-five pairs of somites developed in 

 Tylosurus and only eighteen to twenty in Alosa. (The terms somites 



