THE ADHESIVE ORGANS OF AM1A. I 3 5 



closely apposed, yet they can be traced as distinct layers to a 

 point immediately below the pre-cerebral mass. It is in this 

 locality that the adhesive organs appear. 



In an embryo of ninety hours which covers about 180 degrees 

 of the circumference of the egg the adhesive organs are not ob- 

 servable in surface views but in sections their beginnings are ob- 

 vious. Fig. 2 represents an obliquely sagittal section of the 

 anterior portion of an embryo in this stage. The superficial ecto- 

 derm (s.ec.) consists of two layers of cells which are invaginated 

 just at the anterior boundary of the forebrain (f.b.). The brain 

 cavities are now well defined. There is, however, as yet no in- 

 dication of the infundibular or epiphysial evaginations. The 

 notochord extends nearly to the level of the anterior end of the 

 brain, but owing to the obliquity of the section its anterior por- 

 tion is not represented in the figure. The cavity of the gut is well 

 defined beneath the posterior portion of the brain ; it is greatly 

 reduced in size anteriorly but after reaching the level of the ecto- 

 dermic invagination described above, it again expands into a wide 

 cavity {g.d.). The walls of the gut show little change until the 

 head region is passed when the cells of the dorsal wall change 

 from flat or cuboidal to columnar. These greatly thickened areas 

 of the entoderm form the beginnings of the adhesive organs. 



Transverse sections of this stage (Fig. 3) show that the organs 

 have arisen as paired diverticula of the anterior end of the foregut. 



In an embryo of one hundred and twenty-five hours, covering 

 about 260 degrees of the circumference of the egg the adhesive 

 organs show plainly in surface views as paired structures lying on 

 either side of the median line. An obliquely sagittal section passing 

 through one of the organs is represented in Fig. 4. It will be 

 noted that the lumen of the brain is enlarged and its subdivisions 

 more clearly marked. The dorsal wall of the forebrain now 

 shows a slight evagination which is the beginning of the epiphy- 

 sis ; just opposite in the floor is another evagination which is the 

 beginning of the infundibulum. The foregut is here well shown 

 with its forward extension into the pre-cerebral region where it 

 ends in a wide dilatation (g.d.). The dorsal wall of this cavity 

 is composed of elongated entodermal cells essentially similar to 

 those seen in Fig. 2. Just beneath the adhesive organ is one of 



