464 
DE. ~W. H. BAN SOM ON THE OVUM OF OSSEOUS FISHES. 
temperature. In warm weather they have been noted in six minutes, in cooler weather 
in fifteen or twenty minutes after impregnation. They cause a flattening of one side of 
the yelk-ball, to see which it is often necessary to roll the egg over (Plate XVI. figs. 35 
& 35'). The flat surface gradually becomes a sulcus, giving a reniform outline to the yelk 
(Plate XVI. fig. 36). It then extends all round, giving rise to a dumbbell shape (Plate 
XVI. fig. 37). This sulcus, which may be termed equatorial, travels with considerable 
but variable rapidity towards the germinal pole, producing as it passes on, the flask form 
(Plate XVI. fig. 38). The sulcus is lost by passing forwards to the germinal pole, not 
by relaxation. It is seen for a brief space affecting the thickness of the germinal disk 
only, to which it gives a nipple-like form, while the food-yelk is round (Plate XVI. fig. 39). 
When effaced, the whole yelk-ball is globular and at rest, the germinal disk being no 
longer prominent (Plate XVI. fig. 40). This series of forms recurs with more or less of 
regularity, and with some variations both of time and form, about fifteen or twenty 
times ; each series being the result of a travelling wave. About five waves pass in ten 
minutes. Sometimes a wave commences as usual near the equator, and then for a short 
space passes towards the ventral pole ; but it soon returns, and passing forwards towards 
the germinal pole, is then lost ; occasionally other irregularities occur, such as two or 
even three waves travelling at the same time, a new one having commenced before the 
previous one had ceased (Plate XVI. figs. 41 & 42). This is more often the case in 
warm weather. 
The concentration of the discus germinativus is somewhat greater as each wave comes 
to that pole, although some diffusion occurs again, always as the round form is reproduced 
(Plates XVI. & XVII. figs. 35 to 49 inclusive). The contractions continue, although 
gradually declining in vigour, up to the period at which cleavage begins, after which I 
could not trace them beyond the area of the germinal disk. (See Plate XVII. figs. 44 
to 49 inclusive, which are drawings made at short intervals, until the commencement of 
cleavage ; and show some singular forms of the germinal surface of the food-yelk, which 
are difficult to understand, as contractions of its substance. The figures show also the 
constantly recurring elevation and depression of the germinal disk caused by the travel- 
ling waves.) The periphery of the germinal disk is perpetually varying, being now 
sharply defined, now shaded off and diffused, but it always has a circular outline. 
Coincidently with these contractions, oscillation of the whole yelk-ball takes place. 
At first this is so slow that it requires the use of a cobweb micrometer. As the contractile 
waves increase in vigour and rapidity, the oscillations quicken. 
During the early feeble contractions,' the micropyle, except in cases where from defi- 
cient supply of water the funnel is not quite withdrawn from the pit in the germinal 
disk, has its position changed relatively to the germinal pole of the yelk, by a slight 
imperceptible swing of the latter. 
With the vivid contractions begin visible oscillations of the yelk-ball, so that its ger- 
minal pole swings through about 60°, usually in a plane, which cuts the micropyle, and 
which may be vertical, horizontal, or inclined, but is not a true plane, as the germinal 
pole describes an ellipse. 
