THE ADHESIVE ORGANS OF AMIA. I 37 



The section of the organ represented in Fig. 7 is from a larva 

 in which the head and tail are both free from the yolk. 



The organs are now entirely detached from the gut although 

 the two layers of the entoderm forming the gut diverticula are 

 still discernible. They lie, for the most part, in the ectoderm. 

 They have forced their way through the deeper layers and are 

 covered externally by only a single layer. Even this layer is now 

 very thin and the cell boundaries are no longer well defined. 

 The two organs are now separated from each other and each is 

 subdivided into six to ten separate disks. Fig. 7 represents a 

 section through a single disk. The minute structure of the 

 organ is similar to that previously described. It is composed of 

 pseudo-columnar cells in which the nuclei are situated nearer 

 the bases of the cells than hitherto noted. The basal half of 

 each cell contains some large yolk granules and a coarse reticulum 

 which takes a deep haematoxylin stain. The peripheral half of 

 the cells, on the other hand, is free from yolk but contains a fine 

 reticulum which stains only faintly with haematoxylin. This 

 faintly staining portion of the cell contains minute granules which 

 are best interpreted as prozymogen granules. Although the 

 organ has all the appearances in the present stage, as it had in 

 several of the preceding stages, of a mucous secreting structure 

 it is not yet functional as an adhesive organ. One remarkable 

 change has taken place in the structure of the organ. The ento- 

 dermal cells show a changed polarity. In the preceding stages 

 the most active cell metabolism was in the ends of the cells next 

 to the gut cavity ; now the most active cell metabolism is in the 

 outer ends of the cells next to the exterior of the body, while the 

 earlier clearer ends are now filled Avith fine granules. 



In the larva of 4-5 mm. the organs have broken through the 

 superficial ectoderm and open directly on the surface of the body, 

 although they are as yet partly covered by the ectoderm, as a 

 glance at Fig. 8 will show. The cells of the organ here shown 

 are not different from those described in the preceding stage 

 excepting that the clear zone of the cell appears to extend over 

 one third of its length instead of one half. 



In the larva of 8-9 mm. (Fig. 9) the disk has changed from 

 an oval to a rectangular form as a comparison of Figs. 8 and 9 



