250 B. W. MACBEIDB. 



lightly identify what he means, are only the epithelial lining 

 of the perihsemal spaces which at a later period become closely 

 apposed to the ectoderm. The first trace of muscles in the 

 body-wall appears much earlier. PI. XXI, 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 larvae. 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. PI. XIX, figs. 137, 138, 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 Larvae. 

 I mentioned above that the demonstrative proof that the sac 

 I have termed the right hydroccele is of that nature is obtained 

 from the study of abnormal larvse. I suppose that about one 

 in thirty of the larvse 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.^ 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 



' 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." 



