
540 THE AMERICAN NATURALIST. [Vor. XXXVI. 
from the endoderm, in the nearly related species, Bufo lentigi- 
nosus, the anterior part of the notochord is certainly mesodermal 
in origin. After the mesoderm layer has formed, the endo- 
derm in the mid-dorsal region of the embryo is never more 
than a single layer of flattened cells. There is a closer con- 
nection between the mesoderm and the endoderm above the 
center of the archenteron than at the sides, but I have never 
found an egg in which these two layers could not readily be 

Fic. 22. — Part of a section from the middle region of the same egg. ÆC., ectoderm; ÆW., 
endoderm; M., mesoderm; N., notochord, 
distinguished from each other. The endoderm cells are always 
much flattened at this period, and they invariably contain a 
considerable amount of pigment. The mesoderm cells and 
also the cells of the notochord are larger, more rounded, and 
contain but little pigment in comparison with the endoderm 
cells. » 
In conclusion, I wish to express my thanks to Prof. 
T. H. Morgan for advice and criticism during the progress 
of my work. 
BRYN MaAwR COLLEGE, BRYN Mawn, PA., 
December 16, 1901. 

