DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 697 



a balloon (No. 20a). The change of outline Bambeke attributes to epiboly, the blastoderm 

 squeezing the fluid yolk out of shape ; and this is not improbable, for in various pelagic 

 ova the naked yolk, i.e., that part not yet covered, projects boldly from the blastopore 

 like a plug pressed out from the diminishing aperture. The change in shape 

 might be attributed to the contractility of the yolk — an inherent property according to 

 Eeichert (No. 134) ; but there is much reason for holding that the active agent is the 

 amoeboid protoplasmic cortex, or the blastoderm itself external to that layer. A most 

 remarkable phenomenon was observed by Lereboullet in the ovum of Esox at the stage 

 just referred to, when the usual rotation is perceptibly diminishing, for he states that 

 the blastoderm seemed to continue its rotation " as if disconnected from the yolk, and 

 the latter continued to turn from right to left as though inside a loose sac" (No. 93, 

 p. 491). 



What the significance of these varied movements really is cannot be definitely stated ; 

 but that they are connected with the separation of the germinal matter from the food- 

 yolk proper, as Ransom surmises, seems very probable. Ransom, indeed, would go further, 

 and regard them as a form of contractile movement, not remotely connected with seg- 

 mentation (No. 127, p. 495); and it is noteworthy that these movements cease when the 

 germinal matter has, for the most part, separated from the trophic element in the 

 vitellus. The yolk alters its form soon after fertilisation, as Lereboullet observed in the 

 pike ; and he refers to a movement of the constituent elements of the egg — the marked 

 flattening of the spherical yolk, which now becomes elliptical (see No. 127, pi. i. fig. 17), 

 while the blastodisc projects prominently from its surface. 



Whether the yolk-matter itself, or the protoplasmic envelope outside, really produces 

 the rhythmic contractions referred to, the phenomena depend, as Ransom found, upon the 

 presence of oxygen in the surrounding medium, while carbonic acid produces total cessa- 

 tion or a marked repression of these movements (No. 128, p. 237). They seem to demand 

 less oxygen than cleavage proper (No. 127, p. 495), though the amount of oxygen used is 

 small; and Ransom did not succeed in obtaining chemical evidence as to the products 

 of the oxidation which undoubtedly goes on. 



The conclusion that all the movements collectively known as yolk-contractions are 

 connected with the polar segregation of the germinal protoplasm, is probably near the 

 truth. That their existence, or at any rate their vividness, is correlated to peculiarities in 

 the early development of the germ there is no proof, and Ransom's conclusion is very 

 much at variance, indeed, wholly opposed to the facts, when he says that such movements 

 in Esox and Gastrosteus are connected with rapidity of development (No. 127, p. 495). 

 These forms, instead of hatching in a shorter time than those with slow or indistinct con- 

 tractions, have an embryonic development unusually prolonged, so that the reverse of the 

 above conclusion is really true, viz., that the ova in which these movements are not 

 merely indistinct but imperceptible, are of all forms the most rapid in development, 

 and of such rapidly developing eggs those of the Gadidse and Pleuronectidse are marked 

 types. 



VOL. XXXV. PART III. (NO. 19). 5 U 



