[99] EMBRYOGRAPHY OF OSSEOUS FISHES. 553 



applicable to these cases seems to lie in the physiological relation 

 pointed out as subsisting between the germinal and nutritive matter of 

 the egg, which determines the plane of the first cleavage, but this does 

 not disi)ose of the question why one form with a much smaller egg 

 should be supplied with a much smaller amount of gerndnal matter 

 than another with a much larger egg. The consequences of the com- 

 parison just instituted between the ova of the frog and fish do not stop 

 here, however, for we actually find that the amount of germinal and 

 nutritive matter in proportion to each other is not the same even in 

 different genera and species of fishes. I now call to mind illustrations 

 of this from amongst the Salmonoids. In Osmerus the germ is much 

 larger in proportion to the yelk than in Salmo. Of two forms belonging 

 to different families, Alosa has a relatively much larger germ than Ty- 

 losurus, in the latter of which the germinal disk is reduced to tbe ex- 

 treme of relative dimiuutivene^s amongst the fish ova which I have 

 studied. 



The remarkable phenomena which have been discussed in the pre- 

 ceding paragraphs are due altogether to the inherent motility of the 

 protoplasm of the germ and nucleus. What the nature of the force is 

 which impels the protoplasm of the fish ovum to migrate towards the 

 germinal i)ole or to aggregate iuio a germ at all we cannot say. While 

 it exhibits actual contractility and a self-moving i)ower resident in aud 

 manifested by its own substance, science is not yet ready to assert that 

 it knows anything of its efiicient cause. This is a iihenomenon as in- 

 scrutable in its essential nature as the movements of the living amoeba. 

 The movements of the protoplasm of the egg of the fish in the act of 

 forming the germinal disk only resemble to a certain extent those of 

 the amoeba ; there is no exact parallelism. The amceba, in the active 

 stage, moves about continually in its own peculiar way like any other 

 perfect animal ; practically, the protoplasm layer of the fish ovum stops 

 moving in a distinctly amoebal manner after it has aggregated itself 

 into the germinal disk. In these respects the perfect organism of the 

 amosba differs from the germinal matter of the fish no less than in its 

 want of any power of segmentation and metamorphosis into a deter- 

 minate species of fish embryo. The current ridiculous aud unscientific 

 statement that the germs of animals may be likened to an amoeba has 

 no foundation in observed fact. From their earliest incipiency fish ova 

 differ radically from an amoeba in appearance, and would not be mis- 

 taken for one by the merest tyro. 



The nucleus of the amceba after the ingestion of food is usually dis- 

 placed to a marked extent from the central position. This eccentric 

 position of the nucleus of the amoeboid ProtoplaMa seems to be depend- 

 ent upon essentially the same cause as the displacement of the nucleus 

 in the meroblastic egg, viz, in consequence of the absorption of mate- 

 rials by the eudosarc, which require to be raised to the condition of the 

 living protoplasm of the rest of the animal by metabolic processes. The 



