Monthly Microscopical"! 
Journal, Oct. 1, 1869. J 
of the Ovum of the Pike. 
189 
dentation : this indentation travels round the yelk and completes 
the circle. This zone of indentation, which is equatorial, and in 
the median line, travels onwards towards the germinal pole, the 
yelk assuming successively the outline of a dumb-bell, a flask, and a 
sphere surmounted by a little cone. By means of this indentation, 
the lateral half in which it first occurs is rendered less weighty 
than the other half, and hence it rises. By this means a swinging 
movement is set up, whereby the germinal pole moves from side to 
side, but never passing so low as the horizontal axis of the egg- 
shell. At first the germinal mass (carried by the yelk) moves 
almost exactly in a straight line, to and fro ; by-and-by, the mass 
moves in a widely-elliptical orbit around the north pole of the 
vertical axis of the egg-shell, not dipping below an angle of 45°. 
This oscillation takes place now from east to west, now from 
west to east : a slight interval of time separating the two move- 
ments. Yariations frequently occur. This oscillation goes on so 
long as the egg is in vital activity, up to the moment when * 
hatching takes place; and by this rotation the aeration of every 
part of the embryonic surface by the water around is more com- 
pletely ensured. (Fig. 5 represents the situation of the orbit in 
which the germinal mass moves around a, the north pole, seen in 
face.) These movements of oscillation are truly wonderful to 
witness : the yelk mass moves of itself in an orderly, regular, 
almost circular round, within the immovable egg-shell. 
We spoke of the germinal mass last, as a circular disc, in a 
state of subdivision. This subdivision goes on until the subdivi- 
sions are very small. These cells, as we may call them, aggregate 
towards the germinal pole of the yelk, and form there a dark and 
projecting mass. The cells on the outside or periphery of the disc 
multiply and increase, until the surface of the yelk is covered by 
their growth. There is, then, a thin layer of ceils spread over the 
yelk, with a central denser portion of a somewhat globular form, and 
consisting of cells larger than those of the periphery. In my ob- 
servations the germinal matter had increased by cell multiplication, 
so as to nearly cover the whole surface in thirty hours after fertiliza- 
tion. On the third day I saw that this mass was divided into two 
parts by a longitudinal furrow. This is the first rudiment of the 
embryo, and is known as the " primitive groove." I next observed 
two straight faint lines, parallel with the primitive groove, extending 
the whole length of the furrow, marking out the notochord," or 
"chorda dorsalis," and also the spinal cord. It is the rudiment of 
the spinal column. Above this, or "dorsal" to it, is formed the 
spinal cord : below it, or " ventrally," the nutritive system, heart, 
viscera, &c. These lines start from one point, and run round the 
yelk until they reach a point diametrically opposite to the com- 
mencement : each termination is surrounded by an outspread layer 
