400 RICHARD EVANS. 



they do not increase in size, which they would iiave to do 

 were they to become the nuclei of the surface layer of cells. 

 For these reasons the flagellated cells cannot possibly give 

 rise to the flattened epithelium of the lower surface, where 

 the nuclei of the cells forming it are many times as large as 

 those of the surface layer of the larva (fig. 16 a). 



The cells with granular nuclei not only give rise to the epi- 

 thelial layers of the upper and lower surfaces, but also produce 

 the marginal membrane, which diff'ers considerably in thickness 

 and compactness according to the length of time which has 

 elapsed since the fixation of the larva took place. At first 

 only a few cells are seen to creep outside the limits of the 

 body of the larva which is in the act of fixing itself, but the 

 number of cells that wander out seems to increase with a won- 

 derful rapidity. In a short time they present the appearance 

 shown in fig. 23, in which the cells have not as yet arranged 

 themselves so as to form a complete layer, for there are large 

 spaces to be seen between them. In their outward course they 

 seem to struggle on, passing over and across one another. The 

 outer margin of the as yet incomplete membrane is quite irre- 

 gular, and the cells which form it appear to be absolutely in- 

 dependent of their neighbours, the limit of each cell being well 

 defined (figs. 23, 23 a). 



The cytoplasmic mass of the cell body is exceptionally clear, 

 and presents the appearance of an alveolar structure in which 

 the meshes are slightly elongated in the direction of motion. 

 Pseudopodia are not always produced by these cells as they 

 move out ; in many cases they present at least a complete and 

 uninterrupted margin. Sometimes the cells, as they creep out, 

 carry with them some of the flagellated cells, and in many 

 cases they contain yolk bodies. 



As the fixed individual becomes older the marginal membrane 

 becomes thicker and more compact in structure. It now consists 

 of two or three superposed layers of cells, save at the extreme 

 margin, where it is only one cell thick. The outer margin is 

 still quite irregular in places, owing to the continued outward 

 movement of the cells which constitute it; but later on this 



