138 A. C. EYCLESHYMER AND J. M. WILSON. 



will show. The ectoderm (s. ec.) has undergone considerable 

 thickening and differentiation. It no longer extends over any 

 portion of the organ and as a result a far greater number of cells 

 now reach the surface. The character of the cells is somewhat 

 different as a result of the general change in the form of the 

 organ. They are now typical, pseudo-stratified columnar cells 

 while in preceding stages the outer ends were compressed. The 

 cells no longer possess yolk granules while the prozymogen 

 granules show more plainly in the outer ends. That some sort 

 of a mucous secretion is furnished by these cells is indicated, not 

 only by the cellular structure but also by the fact that when the 

 larva? are detached from the various objects, to which they now 

 adhere, it is often observed that fragments of aquatic plants cling 

 to the organs. 



The larva of 13—14 mm. begins to move freely from place to 

 place. It rarely attempts to attach itself as in the preceding 

 stages. Corresponding with this change in the behavior of 

 the young fish there is a marked change in the position of the 

 adhesive organs. When viewed from the surface, some of the 

 disks forming the horseshoe are still distinctly visible while 

 others are barely discernible. All, however, show a great reduc- 

 tion in number when compared with the larva of 8—9 mm. A 

 section of one of these organs is shown in Fig. 10. It will be 

 noted that the entire organ has now sunk below the level of the 

 epidermis and is partly surrounded by dermal pigment. The 

 organ is still in communication with the exterior through a 

 funnel-shaped opening in the epidermis. 



In the larva of 18—20 mm. the organs have sunk still deeper 

 below the level of the epidermis. Some of them still communi- 

 cate with the exterior, while others are completely covered by 

 the epidermis. Those that communicate with the exterior show 

 a long narrow epithelial tube with numerous branches or diver- 

 ticula at its inner end. The walls of these tubes are several 

 layers of cells in thickness as shown in Fig. 11. Toward the 

 inner end of the tube the epithelium becomes thinner until there 

 remains but a single layer of cells covering the smaller branches. 

 This layer becomes broken up in the smallest branches and 

 these epithelial cells become swollen, the nuclei show chromato- 



