^o"u\Ymi.^oS^^^^^ of fJie Ovum of the Fihe. 193 
sule, or ear ; c, the point where the arterial current runs on to 
the yelk.) Very soon the arterial current, instead of running on 
to the yelk at c, passes farther along under the vertebral column, 
makes a very acute bend, and returns by a vein to the yelk (Fig. 18). 
This prolongation gradually lengthens until the tail-end is reached. 
11.45. — Fish, as is well known, breathe by means of gills, or 
branchi.T. These consist of a multitude of very fine blood-vessels, 
which lie freely exposed to the surrounding water. At the period last 
indicated I saw the first rudiments of the branchial apparatus appear. 
The position of the branchial arteries is represented in Figs. 19 
and 20. Fig. 19 : a is the as yet undivided primitive artery, and h 
one of the branchial arteries. Fig. 20 : is a branchial artery repre- 
sented in position, the one of the other side being unseen ; e being 
the heart. Fig. 20 represents a stage more advanced than the 
present one, but is introduced here to show position of the branchiae. 
At this period, 11.45 p.m., I observed a structure (marked by cross- 
hatching surrounding letter d, Fig. 20), obhquely crossing which 
subsequently became developed the branchial arteries and the sup- 
porting arches. At this time I noticed the division of the main 
trunk into left and right, the left curving upwards and around the 
rudimental branchial structure running underneath c, the auditory 
capsule, to join its fellow of the right side (not seen). This right 
trunk passes underneath the head, makes a similar curl upwards 
and backwards, and, in fact, strictly corresponds to the left one. 
These may be called the right and left primitive trunks. The heart 
now appears as a thick walled tube, dilated into posterior and 
anterior chambers, the former being the larger. The anterior is 
curved, and the valvular action between it and the posterior con- 
sists of an inflection of the wall, whose outside edge is concave, into 
the cavity of the other wall, fitting like a knee into the hollow, and 
so shutting off the current. Fig. 21 : a, posterior chamber ; 5, 
anterior, with the knee-like valve. Pulse now 90 per minute, being 
a fall of 14 per minute since 12.30 (11^ hours ago). Blood- 
corpuscles are being detached from the yelk surface, where they 
had their origin, and pass to the opening of the heart. Fig. 22 : 
a, the right eye ; h, posterior chamber — the right primitive trunk is 
seen passing underneath the head ; c, the thin line, marks off a 
space around the heart, a receptacle for the blood before entrance 
into the heart, which I have called the prsecordial area; d, the 
egg-shell. The prsecordial area is marked out by a white immov- 
able rim ; corpuscles pass through certain gaps which remain con- 
stant, as the corpuscles may be seen to pass along the rim for a 
little distance to reach an opening. Before reaching the rim they 
advance at a uniform rate ; after they have passed the boundary 
they are repelled slightly with each beating of the heart. There 
are three currents seen from the left side entering the heart 
VOL. II. p 
