DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 695 



Especially is this the case in ova which show a preformed discus proligerus. In the pike, 

 for instance, Lekeboullet states that both are alike, save in the formation of a " disque 

 huileux " which collects in the fertilised egg, as Gerbe also describes in the egg of the 

 trout, two hours after fertilisation, the circular germinal area appearing as if enclosed 

 in a "crown of oil-globules" (No. 57, p. 330) ; yet even this feature may appear in the 

 unimpregnated egg, and it cannot therefore, as Lereboullet confesses, be traced to the 

 action of the sperm (vide No. 93, p. 478). The fertilised ovum in pelagic forms (e.g., cod 

 and gurnard) is more readily distinguished, as the segregation of the protoplasm is plainly 

 visible within an hour or two after fertilisation; but the transference is not to the upper 

 pole, as in a large number of demersal forms, but to the lower pole, where the patera or 

 flattened disc is formed of clear, straw-tinted protoplasm containing minute spherules, 

 which are especially numerous at the base and periphery. During the process of segrega- 

 tion the contour of the vitellus becomes very distinctly corrugated — an appearance pro- 

 duced by the streaming of the protoplasm along definite meridional lines ; and pelagic 

 forms are especially favourable for observing this polar transference. Ransom, in common 

 with other observers, wholly failed to detect this movement (No. 127, p. 458), though he 

 says that the granules often form radial lines round the margin of the concentrating disc 

 (Ibid., p. 459). Besides passing along the superficial areas, much protoplasm probably 

 also glides in the deeper strata of the vitellus to the base of the germ during the first 

 hour after entrance of the sperm. Such streaming of the protoplasm towards the disc has 

 been noted by many observers, and recently Kowalewsky has described it in Carassius, 

 Polyacanthus, and Gobius (No. 86). In two hours or more, according to the temperature 

 and other conditions, a plano-convex disc is formed, composed of an almost homogeneous 

 matrix. The disc in the fertilised ovum is always well defined and prominent, and 

 continues to receive additions of protoplasm, so that it increases in size, and becomes more 

 pronounced; whereas in the unfertilised ovum, when a disc is formed, it becomes "vague, 

 irregular in outline, and loses coherency" (No. 57, p. 330). The primary segmentation- 

 nucleus has rarely been detected in the blastodisc before cleavage, granules and colourless 

 vesicles alone appearing in its matrix. The breathing chamber gradually becomes more 

 distinct ; but this may also happen in the unfertilised condition, as Ransom found that such 

 ova may, after being in contact with water for an hour, show this marked interspace. Its 

 formation, as well as the concentration of the disc, Ransom holds to be only indirectly 

 due to the spermatozoa, which may render more easy and rapid the influx of the 

 surrounding medium into the egg (No. 127, p. 463). The same observer carefully studied 

 the formation of this space in Gastrosteus, and states that it first appears close to the 

 micropyle, whence it " gradually extends over the rest of the yolk-ball, being complete in 

 three to five minutes after the spermatozooids have been applied" (Ibid., p. 457); but in a 

 note at the foot of the page he says that water may enter more freely, and the chamber 

 arise simultaneously in the ova of other fishes. Newport, who was the first to signalise 

 this perivitelline space, speaks of it as " respiratory," and being in Rana " at first but a 

 small area" (No. 112, p. 187), a view coinciding with Ransom's upon the same ovum, for 



