138 MICHIGAN ACADEMY OF SCIENCE. 



adhesive organ. I-ts walls are composed of a single lajer of high 

 columnar cells, which are in contact with the two-lavered ectoblast. 



In slightly older embryos the crescent shaped area has given place to a 

 pair of hemispherical protuberances which are quite as prominent a 

 feature of the head as the optic vesicles immediately in front of which 

 they lie. In the median line between the two protuberances and directly 

 in front of the tip of the forebrain is a smaller protuberance, or button- 

 like elevation, which is a remnant of the middle part of the crescentic 

 area of the preceding stage, while the two, large paired protuberances 

 are developed from the horns of that area. They each contain a sac-like 

 cavity, which opens widely to the foregut and the cells of the walls 

 are higher and more columnar than before. We now have the funda- 

 ments of each of the halves, or U-shaped ridges of the adhesive organ. 

 The original diverticulum has become divided into three diverticula, a 

 small median and two large lateral ones. 



In somewhat older eggs in which the embr^-o extends over about 

 220 degrees of the circumference and in which both the head and tail 

 are protuberaiit, the adhesive organ has the form of two U-shaped ridges 

 which lie at the very end of the snout in contact with the optic vesicles 

 and with their concavities directed toward one another and toward the 

 median plane. In fact, they have nearly the position of the ridges in 

 the organ of the newly hatched larva? described above. The median, 

 button-like elevation is no longer visible. Internally it is found that 

 the diverticula or the paired protuberances of the preceding stage have 

 become extended and have taken on the form of long, curved tubes 

 which open widely as before into the foregut. The cells of these diverti- 

 cula are directly continuous with those of the foregut; they are more 

 columnar than before and their ends which lie toward the lumina are 

 clear, while the opposite ends are tilled with yolk granules. This stage 

 differs from the preceding, mainly in the fact that the paired diverticula 

 have become U-shaped and that the button-shaped elevation has dis- 

 appeared. 



In embryos a very little older than the one just described, no external 

 changes are seen, but sections reveal the fact that the lumina of the 

 diverticula, which are still in connection with the foregut, are divided 

 into alternate wide and narrow portions, so that they present a beaded 

 appearance. The six to eight dilatations, or wider portions, are the 

 fundaments of a series of spherical, closed vesicles, which later give 

 rise to the open cups of the functional organ. The walls of the diverti- 

 cula continue to be composed of but a single layer of columnar cells in 

 close contact with the very thin ectoblast. No mesenchyme intervenes 

 between the ectoblast and the fundament of the adhesive organ. 



The changes which lead from this stage to that of the hatching stage 

 first described, follow each other closely. First: Each of the dilatations 

 becomes independent and forms a closed hollow sphere or vesicle. 

 Those in connection with the foregut are also cut off and the several 

 vesicles lie separate, but close together and in such a manner as to 

 form two U-shaped ridges. The external appearance is as described in 

 the previous stage. Second: The cavities of the vesicles shift their 

 jiositions so that they lie against the external ectoderm of the snout. 

 The wall of each vesicle is now no longer of the same thickness at all 



