68 Miscellaneous. 
of the young branchia; a space is reserved along its two margins 
(the external and the internal), in one of which will be formed the 
efferent vessel, and in the other the special gland of the branchia. 
One of these undulations, considered in its totality, may be re- 
garded as a semicircle formed by three parallel curves of cells, a 
middle one enclosed between an external convex and an internal 
concave one. Supposing the two extremities of this arc fixed in the 
same plane, if growth took place with equal rapidity in the three 
layers of cells, we should soon have a large cul-de-sac, no longer a 
semicircle, but more or less conical and deep; but things go on 
otherwise—the cells of the middle layer increase in number, and 
push before them the epithelium forming the convex surface, while 
that which forms the concave layer is not modified. By advancing 
more and more by means of a terminal focus of cell-division, the 
median layer gives rise to a lamina clothed on its two faces by the 
convex epithelium. The cells of this lamina, which are at first con- 
tiguous, soon separate from each other, so as to form lacunee, and, at 
certain points, vessels, From this it results that, as this process 
is repeated alternately to the right and left of the primary undulated 
lamina, we obtain sections of the branchia composed of a slightly 
undulated axis, from which issue, to the right and left alternately, 
lamine which become longer and longer the further we go from the 
extremity of the branchia. A little later we easily distinguish a 
small muscular band, which follows the inferior margin of each of 
the laminze composing the branchia and fixes it. 
Each of the laminw formed as I have just stated produces in its 
turn a series of undulations by becoming folded in the direction of 
its width. But in this case the undulations are much hollowed, 
and correspond with strong eminences on the other side; no new 
productions are formed at the expense of the median layer, which 
remains even throughout and preserves its two epithelia. These 
undulations start from the point of attachment of the lamina, to 
run, gradually diminishing, to the point where is the focus of in- 
crease and where the new folds are formed. 
Lastly, in the adult we observe a third system of undulations, 
consequently of the third order, situated perpendicularly to the point 
of inflexion of the laminz, the formation of which I have just 
described. These series of new folds only appear very late in the 
embryo; at the moment when, being on the point of quitting the 
egg, it measures about 15 millim. in length, we only see scarcely 
perceptible traces of them, but they become quite distinct when in- 
jections of the branchia are made, which, however, is a very delicate 
operation. 
As regards the vessels of the branchia, the one which conveys the 
blood to it appears early at the commencement of the formation of 
the laminz ; it occupies nearly the centre of the organ, and is com- 
prised within the base of the lamine and the gland of the branchia, 
which is also distinctly marked at this period. The efferent vessel 
is formed upon the crest of the branchia and on the outer border of 
the lamine ; it is undulated, like the parts which bear it, and issues 
