DR. W. H. RANSOM ON THE OVUM OF OSSEOUS FISHES. 
469 
applications of the current were made, but they were only followed by slow shrinking 
of the remaining pouches of inner sac, by which the food-yelk and even the oil were 
squeezed out, and the whole mass at length became darkly granular (Plate XVIII. 
fig. 64). 
I afterwards ascertained that one application of the current, sufficiently strong to 
cause a deep indent, was generally followed after a time by rupture of the inner sac, and 
all the further changes above described. Repeated applications of the current hastened 
them however. 
It was somewhat difficult to adjust the strength of the current, so as to excite well- 
marked contractions, and yet not cause rupture ; however, by using the smallest amount 
of metallic contact requisite to put the machine in motion, by pushing the coil quite off 
the core, and by closing the circuit for only two to five seconds, I obtained trustworthy 
results. 
With these, which may be called zero-currents, I excited peristaltic waves, distinguish- 
able from the normal ones by their greater depth, abruptness, rapidity of formation and 
of progress, by the varying directions in which they travelled, and the positions at 
which they originated. An excited contraction may begin near to either electrode, or 
distant from both ; the sulcus may be in the direction of the current as in Plate XVIII. 
fig. 65, or at right angles to it, as in Plate XVIII. fig. 66. Such waves may be 
equatorial or meridional, as the same figures show. The zero-currents cause no ob- 
servable electrolysis, and are slower in exciting the contractions than the stronger cur- 
rents are. 
The position of the rupture in the inner sac varied much ; it had no constant relation 
to the electrodes, or to the poles of the yelk-ball. Sometimes it took place near to the 
indent, especially if the current was strong ; sometimes at the part of the yelk-ball most 
remote from the contraction, and then it was preceded by a protrusion and distension of 
the inner sac, which exhibited a marvellous extensibility. 
Ova which had arrived at that stage, when, being about to cleave, their natural con- 
tractions had nearly ceased, contracted in a similar manner, but required perhaps some- 
what stronger shocks, and the interval which elapsed between the application of the 
galvanism and the commencement of the contraction was rather longer. Ova in the 
second stage of cleavage, when the normal contractions had ceased, were markedly con- ' 
tractile on the application of moderately strong currents. . Unimpregnated, ova, when 
submitted to a moderately strong current, soon imbibe water, form a breathing-chamber 
and contract ; and then rupture of the inner sac and electrolytic changes are very apt 
to occur near the electrodes. _ ■ 
The excited contractions, although, like the normal ones, they began almost con- 
stantly upon some part of the outer surface of the food-yelk, extended afterwards to 
that surface which lies in contact with the germinal disk. 
Neither the germinal disk, nor the separate cleavage masses could be made to exhibit 
any contractile movements by galvanic irritation, although certain changes of their form 
