56 CAUDAL GILLS OF ZYGOPTERID LARV^, 



enclosed within the fused network formed by the cells. These 

 are the first alveoli. In Text-fig.2, I have attempted to show 

 how the formation of the alveoli is brought about. 



It would be difficult to decide the question as to the exact 

 composition of the alveolar wall, were it not for a fortunate 

 circumstance in connection with the fixation of the hypoderm- 

 layer. Within the gill, the hypodermis carries on its inner 

 margin a tine basement-membrane, which, luckily, becomes 

 detached in places, either by the action of the fixative, or per- 

 haps during sectioning. This delicate membrane closely resem- 

 bles the innermost stratum of the cuticle before it becomes 

 hardened up Now, in many places it can be seen that the 

 alveolar wall is continuous with this basement-membrane, and 

 there is no change in structure as we pass from the basement- 

 membrane proper into the interior of the gill. 



Again, if we examine the nuclei upon the alveolar walls (Plate 

 iv., fig. 30), we find them to be exactly similar in size, shape, 

 staining qualities, and contents, to the hypoderm-nuclei found 

 along the epithelial layer. We must conclude, therefore, that 

 the alveolar nieshwork is the product of numerous hypoderin-rells 

 tvhich have grown into the interior of the gill^ and havf. enclosed 

 within their folds a nnTYiher of spheroidal chambers (alveoli) rvhose 

 interiorfi were originally portion of the hoimocvele. 



The fact that blood-coagulum, with an occasional amoebocyte 

 (but never, as far as I have seen, any miocytes) is often seen 

 within the alveoli, bears out the above statement. Jf the 

 alveolar cavity were the interior of a single cell, how indeed 

 could blood-plasma become enclosed within it? But if the 

 mesh work grows, as I have indicated, by tiie branching and 

 fusion of numerous cells throughout the hsemocoele, then it 

 becomes a certainty that some at least of the blood must become 

 enclosed within it. As the miocytes continue to travel regu- 

 larly in the course of the main blood-circulation, which becomes 

 closed off, as the mesh work grows, in the form of two blood- 

 canals, it is not surprising that we do not meet with these 

 corpuscles in the alveoli. Even if one were accidentally enclosed, 

 we may be fairly sure that it would cease to keep its oat-shaped 



