192 Ohservations on the Development [^rZL.^cTf'ff 
tion. The posterior chamber is seen to close-to, like two flat 
boards coming together ; then the anterior one contracts, the con- 
traction travelling along, vermicularly, as it is called, or with a 
movement like that seen in the locomotion of the earthworm. 
(Fig. 15: a, the posterior, h, the anterior chamber; one of the 
figures shows the flap-like sides of the posterior chamber, the other 
the circular constriction between the two cavities.) It is interesting 
to note that at the same time that contractions occur in the heart 
for the first time, they also occur in the muscles forming along the 
back of the embryo, which is thus seen to endeavour as it were 
to straighten itself out. 
Coincidently with these changes resulting in the formation of 
the heart, the formation of the whole body is proceeding with 
rapidity. By this time the embryo consists of a head, with brain, 
eyes, and ears, a heart, and a vessel proceeding from it to go down 
the body underneath the vertebrae, a long chain of vertebrae termi- 
nating a little distance from the tail, which latter terminates in a 
moderately pointed manner ; above the vertebral bodies, the spinal 
cord with its median furrow ; these again covered in by muscle and 
integument. The embryo reposing on and laying around the yelk- 
bag, from which the creature derives its nourishment, and on the 
surface of which blood-corpuscles are seen passing to the heart, 
the whole oscillating within the water-chamber enclosed by the 
elastic egg-wall. 
12.30 P.M., 981 hours. — The heart beats 104 times per minute. 
The two cavities contract and dilate alternately ; the posterior 
contracts whilst the anterior dilates, and vice versa. 5.30 p.m., 
103^ hours. — By this time, the tail-half is detached from the yelk 
surface, and is now lying free in the water-chamber (Fig. 16). 
The yelk-sac is diminishing in size, the growth of the embryo re- 
moving the contents gradually by absorption. 10.45 p.m., 108f 
hours. — Circulation of blood seen for the first time. At this ear- 
liest period the course taken by the blood is as follows : — Two 
currents of blood are seen passing towards the auricle, one running 
along the median fine of the yelk surface; the other comes from 
under the body, probably from the other side of the yelk, below 
a structure situated a httle nearer the tail than the heart. These 
may be called veins. The blood in these two currents uniting, enters 
the praecordial space, passes through the posterior and anterior 
cavities, and then, by a large vessel or artery, which immediately 
divides into two, is carried to the under-surface of the ear, making 
a sharp curve ; two vessels pass as far as to the first vertebra, where 
they unite, and the united artery passes underneath the vertebrae to 
a point where the body is disjoined from the yelk. Here the blood 
runs on to the mid-line of the yelk, and reaches the point whence it 
started. (Fig. 17 : a, h, the two currents ; the auditory cap- 
