90 BULLETIN OF THE 
so arranged in pairs that the convex surfaces of all of the nuclei in one 
row are directly opposite the convex surfaces of the same number of 
nuclei of an adjacent row. The cells whose nuclei constitute such a pair 
‘ of rows form the two walls of a thin flat hollow sack, a respiratory 
lamella. The outer surface of each lamella is covered with a continuous 
thin chitinous secretion from these cells. The flattened surfaces of the 
nuclei are turned outwards as regards the lumen of the sack, and the 
convex faces are turned inwards. Ultimately the cells corresponding to 
each pair of nuclei, which thus face each other, come in contact, and are 
apparently fused together, thus forming pillars of protoplasmic substance 
joining the walls of the lamella. The posterior borders of the lamelle 
are free, and in most of the sections a pair of these cells with large nuclei 
are found at these free ends. In the later stages (six or eight days after 
hatching) the marginal pairs of cells become pigmented like the “ hypo- 
dermis” cells generally. The other cells remain up to this time without 
pigment. ; 
The surface of each lamella presents, as has been said, two chitinous 
limiting membranes, one forming its dersal surface, the other its ventral 
surface ; these are continuous with each other at the free (posterior) 
rounded margins of the lamelle, and at their anterior limit with the cor- 
responding membranes of the lamella next above and below, respectively. 
There is a constant difference between these chitinous coverings: that of 
the ventral surface is smooth and of uniform thickness, that of the dor- 
sal surface is early characterized in sagittal sections by the presence of 
fine, close-set faintly expressed tooth-like markings. These markings 
may be traced over the free edge of the lamella, but do not extend on to 
its ventral surface. 
The space embraced between the two chitinous layers of each lamella 
is interrupted at intervals by the short 2-cell columns described above, 
each with a single nucleus, which is so large as to touch its neighbor. 
The protoplasm enveloping the nuclei is #eccodingly scanty, and thins 
out at the flat margins of the nuclei into a layer which it is difficult to 
trace as a lining to the chitinous membrane. It occasionally presents 
slight irregular elevations, which project into the common cavity. 
Through the anterior attachments of the lamellz, the cavities of which 
communicate directly with the body-cavity, the blood has free access to 
the cavity of each respiratory lamella ; blood corpuscles are conse- 
quently to be seen in sections (Pl. XII. fig. 76, cp. he.), and the 
coagulated plasma of the blood often fills the lamellar cavities more or 
less completely. 
