368 E. W. MACBRIDE. 
rightly identify what he means, are only the epithelial lining 
of the perihzemal spaces which at a later period become closely 
apposed to the ectoderm. The first trace of muscles in the 
body-wall appears much earlier. Pl. 28, fig. 145, shows the 
formation of a well-marked muscular band from the wall of 
the right posterior ccelom of a larva of Stage E. We see that 
it consists of indubitable myo-epithelial cells. I have traced 
this band into the oldest specimen I have examined for histo- 
logy ; and so far as I can see it appears to become a dilator 
of the anus. It is very strange that it should appear long 
before any other muscles of the body-wall ; it forms quite a 
conspicuous feature in sections of all well-preserved metamor- 
phosing larve. The same figure shows the first trace of histo- 
logical differentiation in the mesenchyme; we see the first 
formation of that fibrous intra-cellular substance which gives 
_ firmness and tenacity to the adult body-wall. 
The cells of the gut remain unchanged till the very end of 
the metamorphosis, but in Stage G we can trace some differen- 
tiation. Pl. 26, figs. 127, 128, show part of the lining of the 
adult oesophagus and of the pyloric sac of such a larva. The 
cells of the former are very long and narrow, and their outer 
portions take a clear yellow tone with osmic acid; those of the 
latter are ordinary cylindrical epithelium cells. 
Abnormal Larve. 
I mentioned above that the demonstrative proof that the sac 
I have termed the right hydroceele is of that nature is obtained 
from the study of abnormal larve. I suppose that about one 
in thirty of the larvee I examined were abnormal, though in very 
different degrees. The commonest abnormality results from 
the unusually great development of the organs of the right 
side, and the consequent checking of the metamorphosis.1 The 
larva of which the two sections are given in figs. 85 and 86 
had about attained Stage D. The left hydroccele is perfectly 
normal, but the right, though not much larger than usual, is 
1 The reader will remember that in the analysis of the metamorphosis which 
I have given on p. 355, one of the main factors recognised is “the preponde- 
rating growth of the organs of the left side.” 
