188 
Observations on the JDevelojpment 
[Monthly Microscopical 
Journal, Oct. 1, 1869. 
an imperfect cleavage, &c. So that, wlieii I speak of fertilized ova, 
I do not necessarily exclude barren ones. 
A]pril 14:th, 8 p.m. — I did not make any observations nntil ten 
hours had elapsed after fertilization; so that my remarks as to 
what takes place during that time are constructed from what I 
have heard or read. The egg of the pike is a globular body, the 
size of, and much resembling in colour and translucency, a grain of 
cooked sago. It consists of an outer covering, and of a contained 
yolk or yelk. The outer covering is very elastic, and if an attempt 
is made to pierce it with a sharp point, the egg springs away ; so 
that it is almost impossible to make an opening. We will suppose 
that fertilization has taken place, and one of the eggs is placed in 
the field of the microscope. The first thing observed is that the 
egg becomes visibly larger in water, and this is seen to be due to 
an imbibition of water, which enters by absorption through the 
outer wall, separates the yelk from its previous position in con- 
tact with the wall, distends this latter structure, and forms a fluid- 
medium completely surrounding the globular yelk, and wherein 
it can freely rotate. This water cavity serves, doubtless, by con- 
stant change of the water, as a respiratory medium for the yelk 
and the embryo fish that is formed upon its surface. Over the 
surface of the yelk is seen spread out a thin layer of material 
in the state of granules, or very small particles, which layer is the 
structure from which the embryo is formed. At first this layer is 
laid over the whole surface, but collected more especially towards 
a pole termed the germinal pole, which corresponds to the situa- 
tion of the micropyle. (See Fig. 1 : a, the elastic cell- wall ; 
the water cavity between it ; and c, the yelk, covered with a layer 
of granules, which are more densely aggregated towards d, the 
germinal pole and micropyle.) Yery shortly, the granular matter 
collects more markedly at the germinal pole, gradually passes over 
to it almost entirely, leaving the remaining surface uncovered ; and 
the mass forms a projection on the otherwise circular outline of the 
yelk (Fig. 2). This germinal mass, consisting of an aggregation of 
granules, next undergoes cleavage : a furrow forms across it, divid- 
ing it into two parts; then another forms at right angles to the 
first, dividing the whole into four (Fig. 3, where the germinal 
mass is seen in face, and not in contour). Division goes on into 16, 
64, &c., until the mass is minutely divided, and is then in the 
so-called mulberry stage. It is finally broken up to such a degree 
that the surface becomes smooth again. It now subsides, as a cir- 
cular disc, to the under-surface of which is attached a layer of oil- 
globules. 
As soon as the water-breathing chamber is formed, appear the 
phenomena of contraction and oscillation of the yelk within the yelk 
cavity. On the circular outline of the yelk appears a shallow in- 
