Albert C. Eycleshymer and James Meredith Wilson 155 
structures such as these give rise to some of the most perplexing prob- 
lems with which embryology has to deal. . 
In the surface views (Figs. 17, 18, 19) there is a peculiar median 
knob which appears soon after the adhesive discs are differentiated. 
This structure hes between and somewhat anterior to the discs. It has 
been observed, as earlier stated, by Reighard (see Keibel, 03). Sagittal 
sections through embryos of this stage( Fig. 48) show that in addition to 
the lateral evaginations of the fore gut there is a less marked evagination 
of the median wall. The hypoblast in this region is thickened and be- 
comes continuous on either side with that of the adhesive discs. The 
epiblastic pocket behind separates this structure from the anterior end 
of the fore brain and together with the surrounding mesoblast gives it 
considerable prominence in surface views. We conclude that the so-called 
“button ” is nothing more than the strongly evaginated median portion 
of the adhesive organs. 
THE CHorpDA AND HypocHorDa. 
Dean, 96, has described the formation of the chorda in Amia as 
follows: “The notochord arises as in the sturgeon or gar-pike: it sep- 
arates directly (i. e., delaminates) from the entoderm.” We have studied 
the origin of the notochord in many series of embryos, but are unable 
to add much to Dean’s description. In the great majority of cases ex- 
amined the sections show conditions similar to that observed in Fig. 40; 
often the mesoblast has not yet separated from the axial rod as shown 
in Fig. 41. In but one instance have we found anything which would 
lead us to regard the chorda as formed by an evagination of the dorsal 
wall of the archenteron, as is known to be the case in many amphibia. 
We therefore conclude that the chorda is derived by delamination from 
the layers of cells which we have called mes-entoderm. The controversies 
that have been waged over this structure and the failure to homologize it 
in the various groups, especially amphibia and fishes, hold out little 
promise that a definite solution of the problem in harmony with the germ 
layer theory is near at hand. 
The hypochorda, so far as we know, has not been previously seen in 
Amia. It arises from the hypoblast forming the dorsal wall of the gut 
in a stage just prior to the appearance of the pronephric ducts. It ex- 
tends the length of the notochord and presents in sagittal sections ap- 
pearances which lead us to regard it as irregularly segmented. This 
peculiar structure has been observed in many vertebrates and numerous 
suggestions offered as to its significance. By some it has been considered 
