26 
AV. F. I’UKCELL. 
point, namely, that the out-growths or out-foldings are 
accompanied by an invagination of the ectoderm between 
them in the earliest stage. 
Now, in the case of the rudimentary lung-books in spiders, 
as summarised on pp. 17-20, it is evident that the pulmonary 
folds cannot be considered as due to simple ont-foldings or 
in-foldings of an epithelium whose thickness was that of the 
walls of the folds, as is the case in the American Lira ulus 
at least. On the contrary, they arise by a peculiar process, 
which results in the transformation of a very thick but even 
epithelium into a folded one of one half the thickness, but 
occupying the same volume, and unaccompanied, therefore, 
by any out-growth or in-growth at first. 
1 am of opinion that these two modes of forming a folded 
epithelium are not fundamentally different, for the one may 
be readily derived from the other. On the contrary, I believe 
that the method which obtains in the spider is merely an 
abbreviation of some such process as occurs in the American 
Limulus, being the most convenient one for rapidly throw- 
ing a limited area of a very thick epithelium into folds, for 
this could not easily be done by ordiuaiw folding, as the 
breadth of the area in question is only equal to the thickness 
of the epithelium itself. Which of the two methods was the 
original depends, of course, on the thickness of the appen- 
dicular epithelium in the common ancestor, and is a question 
of but secondary importance. The Japanese form, according 
to the description of Kishiuouye, appears to bear some 
resemblance to the spider in the origin of the respiratory 
lamella}. 
'The result of the folding in Limulus and the spider are at 
first practically the same in each case, namely, an undulating 
folded epithelium, and it is only in the subsequent growth of 
the folds that a real difference between the two cases becomes 
apparent. For in each the epithelial cells Tiiultiply by 
division in such a manner that the walls of the folds expand 
and grow, in the case of the spider, into the interior of the 
appendage, but outwards and away from the latter in 
