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 lamellae 

 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 dorsal surface, the other its ventral 

 surface ; these are continuous with each other at the free (posterior) 

 rounded margins of the lamellae, and at their anterior limit with the cor- 

 responding membranes of the lamella next above and below, respectively. 

 There is a constant difi"erence 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. Tliese 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 protoplasrii enveloping the nuclei is exceedingly 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 lamellae, 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. kce.), and the 

 coagulated plasma of the blood often fills the lamellar cavities more or 

 less completely. 



