GERM CELLS OF COELENTERATES 615 



very high powers. A germ cell in this layer would be almost 

 as obvious as in the non-cellular jelly; no evidence of such cells 

 was found. The radial canals, then, remain as the only prob- 

 able path along which cells could pass from the gonad to the 

 tentacle. The shortest path would be along the radial canal 

 which leads to the single tentacle from which the secondary 

 medusae develop; if any other canal were followed there would 

 necessarily be a further migration through the circular canal to 

 the tentacle. In order to canvass the situation thoroughly all 

 the canals were examined carefully throughout their length. 



Figure 40 is a drawing showing the typical structure of the 

 terminal portion of the radial canals and the enlargements at 

 their distal ends. The figure represents the canal and bulb at 

 the base of the single tentacle, but the same structure is typical 

 of all the canals. The entoderm cells are arranged in a single 

 layer in both the canal and the tentacle bulb, their nuclei are 

 rather small and the cytoplasm is much vacuolated. Figure 41 

 is a much enlarged and carefully drawn portion of the bulb at 

 the end of a radial canal; the very large size of the cells, their 

 vacuolated cytoplasm more or less filled with large spherules, 

 and the characteristic nuclei, are well represented. Figure 42 

 is a transverse section of one of the radial canals from the aboral 

 portion of the medusa; the ectoderm layer of the medusa is not 

 shaded but the jelly of the bell and the entoderm cells of the 

 radial canal are shown as nearly like the sections as possible. 

 These entoderm cells, more clearly than those in figure 40, 

 show the character of cell and nucleus, and the structure shown 

 in the figure is typical of the radial canals of all the medusae 

 examined. 



Over fifty medusae were carefully examined through every 

 section of each canal and the conditions found were practically 

 identical with those shown in the figures. There is some vari- 

 ation in the number of cells or nuclei present in any one section, 

 but never was there more than a single layer of cells, never 

 was there any cell crowded back of these canal cells against the 

 supporting layer or jelly, never was there a cell whose cyto- 

 plasm was compact, finely granular and deeply staining as germ 



