C. Watiburton 325 
great regularity in the wall of the oesophagus, in the connective tissue beneath 
the submucosa. How they reach this position is entirely unknown. Carpenter 
(1915) suggests that the larva on penetration may enter a small blood vessel 
and be carried to the oesophagus by the blood stream, but the only basis for 
this supposition lies in the fact that attempts to find the larva near the spot 
where penetration was observed were unsuccessful in the few cases in which 
they were undertaken. The interesting cases, to be discussed later, in which 
Hypoderma larvae have infested man point rather to extensive wanderings in 
the tissues, and it may be that our ignorance with regard to this portion of 
the life-history arises simply from the inherent difficulty of tracing so small 
a parasite. 
The larvae in the oesophageal wall, which usually begin to appear in 
September (in England) and culminate in November, are very different from 
those which hatched from the eggs, being for the most part of a glassy smooth¬ 
ness, their very minute spines being confined to the regions of the mouth and 
the posterior spiracles. They are provided with a median tooth and mouth- 
hooks, and are usually from 6 to 14 mm. in length. There may, of course, be 
larval stages intermediate between this smooth oesophageal instar and the 
spiny first instar which hatches from the egg, and in the case of H . linecttuwi 
one such intermediate form has lately been described by Laake (1920). 
From this point our knowledge of the life-history is fairly complete. The 
oesophageal larvae are always on the move, and may be found with the 
anterior end, pointed in any direction, but whereas they are at first mostly 
situated towards the pharyngeal end of the gullet their incidence progresses 
gradually downwards till towards the time of their disappearance from this 
organ such as remain are found near the origin of the stomach. Again, for 
the sake of comparison, speaking only of what happens in England, and 
neglecting occasional extreme instances, larvae begin to appear in the oeso- 
phageal w all in September, are most numerous there in November, and finally 
disappear in March. 
Now these smooth, glassy larvae, averaging about 14 mm. in length when 
they leave the gullet, are identical with the youngest larvae which are found 
in the subcutaneous tumours. They proceed from the gullet to their final 
situation beneath the hide without change of form or noticeable increase in 
size. There is no invariable route followed in this progress, which takes place 
always in the connective tissue, but many of the larvae find an intermediate 
stopping place in the spinal canal. With the same regularity with which larvae 
are to be found in the oesophagus in autumn, they are to be met with in the 
spinal canal in the winter. 
Probably some if not most of the larvae proceed directly from the oeso¬ 
phagus to the anterior region of the dorsal integument, working their wav 
between the muscle layers. Others have been definitely traced across the 
diaphragm and upwards along the border of the 9th rib to the spinal canal, 
which is entered by a posterior foramen. The canal, where the larvae are 
