494 
DB. W. H. .EANSOM ON THE OVUM OF OSSEOUS FISHES. 
developing embryo, more urgently demand oxygen than do the earlier stages of cleavage, 
and a fortiori, than the yelk-contractions. 
Thus the progress of development and of growth of the embryo ceased as early as 
seven hours after the cell was closed ; and so rapidly had the oxygen been consumed, 
that even the yelk-contraction ceased in eighteen hours, although the proportion of ova 
to water was small : while the earlier cleavage, under less favourable conditions, had con- 
tinued for twenty hours at least, and had not so exhausted the oxygen as to stop the 
yelk-contractions for thirty hours or more. 
The higher the stage of development at which the embryos used for experiment had 
arrived, the sooner did they so exhaust the oxygen as to arrest the yelk-contractions. 
In young embryos in which no striated muscle was seen, the trunk movements per- 
sisted as long as the yelk-contractions, and the heart, which is but a mass of protoplasmic 
balls, did the same, but the striated muscles in the trunk of more advanced embryos lost 
their contractility more rapidly when oxygen was withheld. The heart, even in hatched 
embryos, continued to contract longer than any other structure, perhaps, because it 
continually helped to renew the medium around it. 
Concluding Remarks. 
The observations detailed in this communication seem to me to be confirmatory of the 
view that the egg of osseous fishes is a cell, and to be looked on as a structural unit, 
the prototype of those units which, variously aggregated and modified, form the mass of 
the higher organisms. The minute, simply constructed, early ovarian ovum would, I 
doubt not, be accepted as such by most observers ; and the larger ripe egg can scarcely 
be held to differ essentially if its mode of growth and development be considered. 
The complex structure of the yelk-sac cannot be urged with much force against this 
opinion, as analogous structure has been met with in other parts, which are admittedly 
cell-walls or their descendants ; for example, the striations in the matrix of some carti- 
lages, and in the surface-layer of intestinal epithelium, and the so-called pore-canals in 
the cuticular tissues of many lower animals. It is certainly somewhat difficult to con- 
ceive what is the true position of the egg if not that of a cell. 
Assuming that the opinion now advocated is sound, the question as to the mode of 
growth of cell-walls receives some additional light from the evidence here brought 
forward, to prove that the yelk-sac grows interstitially, and not by accretion upon either 
surface, or by gradual transformation of the surface of the yelk-ball. 
To the yelk-ball, however, as the essential cell, the greatest share of interest attaches, 
and it, like the cell contents, alone is capable of undergoing multiplication. A conve- 
nient definition of a cell-wall might therefore be, — the first separable covering of the 
protoplasmic mass, which does not take part in multiplication by fission. 
The cell-wall must be considered as a living substance, at least so long as it continues 
to grow interstitially, although it is probable that a time occurs in the life-history 
of most cells, which possess such walls, when they cease to grow, when they render 
