u 
MONICA TAYLOli. 
in tlie fin is differentiated earlier than the bone of the 
shoulder girdle. I have not observed any trace of cartilage 
in the development of the girdle. 
Tlie unicellular glands already described are visible exter- 
nally under a lens. 
Three main blood-vessels are distinctly visible in the fin, 
one afferent supplied from the dorsal aorta which runs into 
the fin dorsal to the “ skeleton,” and which breaks up into 
capillaries, and two efferent. These efferent vessels make 
their appearance in the circumferential regions of the fin. 
One of them then takes a route dorsal to the ‘^skeleton,” the 
other ventral. Both pass into the body proper anterior to the 
nodular end of the cartilaginous skeleton and to the shoulder 
girdle, and having joined, they pass backwards parallel to 
the inferior jugulars to open into the heart at a little distance 
behind the junction of the duct of Cuvier with the heart. 
This backward course is due to the fact that the position of 
the heart has been changed relative to the fins since the veins 
were first formed. 
The innervation and muscularisation of the fin has been 
Avorked out by reconstructions made by the Graham Kerr 
method ( 4 ) from sagittal sections. In the earlier stages trans- 
verse sections are useful. The occipital arch Avas taken 
as a fixed point, myotonies and nerves behind this arch 
being numbered 1, 2, 3, etc. It can be shoAvn that spinal 
nerves 1, 2 and 3 innervate the fin. This is an indirect 
method of proving that it Avas the ventral process of the first 
three trunk myotomes that Avere concerned in the formation 
of the fin, but as late as Stage 26 there is no brachial plexus — 
each nerve enters the fin quite independent. 
In Stage 28 the ventral ganglia of the first and second 
spinal nerves, which, unlike the other spinal nerves, were not 
separated by neural arches, are fusing one with the other. 
This fusion, Avhich is probably correlated with the lengthening- 
out process of the branchial cavity already alluded to, has gone 
still further in Stage 29, though the double character of the 
apparently first spinal nerve is still evident. The brachial 
