194 Observations on the Development [^j^Zl\!^oclXim^^ 
(marked by three arrows in Fig. 20) : one (i) from the left side of 
the yelk surface ; another (ii) from the neighbourhood of the mouth, 
entering anteriorly (and coming from the right side of the yelk 
surface) ; the third (iii) from the position of the branchial structure, 
entering posteriorly. The optic capsule (a, Fig. 22) is becoming 
lengthened and curved on itself, so as to form a U-shaped body ; in 
the concavity of this the circular lens is formed. The olfactory 
capsules, the organs of smell, are visible (the two oval bodies at e, 
one on either side of the middle line) as small depressions in the 
snout ; / is the nervous cord which represents the left side of the 
brain at the present stage. It is seen to unite with its fellow in 
front, and between the two is a cavity called a ventricle. There 
are strong movements of flexion in the back, the head bending 
towards the tail and the tail towards the head. 
19th April, 6th day, 12.15 noon. — Looking at the right side of 
the embryo, the right primitive trunk is seen crossing from the left 
side, making a sudden turn upwards and pursuing a curvilinear 
horizontal course underneath the organ of hearing, as on the left 
side. Fig. 23 : c, same figure ; a, ventricle of brain ; h, folding-in 
of the cerebral cord to form a lobe, the middle or mesocephalic lobe ; 
d, the commencement of the left primitive trunk ; e, the prsecordial 
area — in front and behind this letter are the two currents mentioned 
as coming from the right side of the yelk surface (Fig. 20). The 
veins from the head on either side coalesce, and take the course 
shown (for the left side) in Fig. 20, running from the eye, passing 
above the auditory capsule, then descending, arrive upon the yelk 
surface in the praecordial area. On the right side (Fig. 23) it passes 
under the head. At ^ (Fig. 20) this current and the venous supply 
from the left side are seen uniting, to pass on to the heart. The 
venous and arterial currents cross each other, as is seen at /, Fig. 20, 
the artery being underneath. This crossing has led some errone- 
ously to suppose that the two currents, venous and arterial, united. 
Fig. 24 is a diagram of these currents : a, the cardiac opening ; 
(i) right, (ii) left, yelk current ; (iii) left head venous current ; (iv) 
right ditto. The head is attached to the yelk surface by a broad 
band {h, Fig. 20), and it is posteriorly to this that the current from 
the riglit side passes. The optic capsule, mentioned at 11.45 p.m. 
of 18th, as a U-shaped body, has now the two ends united, so that 
the lens is encircled (h, Fig. 20). 
20th April, 7th day : 1 a.m., midnight. — A current of blood is 
seen to pass down the aorta, under the vertebraB, nearly to the 
end of the tail. The aorta terminates by a small vessel, turning on 
itself, in a very large vein : the blood runs along the vein to the 
extremity of the yelk nearest to the tail, and is emptied on to the 
yelk surface ; there being a vein or venous channel on each side of 
the yelk, which runs on its border to the heart (d, Fig. 18). 
