nuclei; 2° the natural differentiation of the two primitive tissues and 
their derivatives occasioned by difference in the quantities of yolk 
they contain; 3° the retarded activity of the endoderm cells, contri- 
bute to render Necturus an especially favorable object for the 
determination of the ectodermic origin of the head cartilages. : 
Before the closure of the neural folds, the ectoderm becomes 
thickened in two broad bands parallel to the long axis of the embryo, 
and extending backwards from the anterior limits of the medullary 
plate. Dorsally each band reaches nearly to the summit of the neural 
fold. Each is thickest near the dorsal wall of the mesodermic somites, 
and fades away as the ectoderm passes off laterally over the yolk. 
Later, the anterior, or cranial portion of the band breaks up into 
a series of vertical ridges, each of which gives rise to a sensory epi- 
thelium followed by a region of cell proliferation in three directions: 
medianly to meet pockets from the endoderm with which the ectoderm 
fuses to form the gill clefts, and laterally into each of the adjacent 
gill arches, where the cells thus proliferated form dense masses with 
peculiarly round nuclei, readily recognized as the embryonic tissue 
from which later the cartilages of the gill arches are formed, 
and through which the blood spaces soon cut their way. Below the 
region in which the fusion with the endoderm takes place, the lateral 
proliferation of mesoderm into the branchial arches continues. We 
may thus distinguish in the ectodermic ridges three regions following 
one another dorso-ventrally as follows: 1“ the region of the sense 
organs; 2 that of fusion with the endoderm and proliferation of 
mesoderm ; 3% that of proliferation of mesoderm alone. 
In the neighborhood of the nasal epithelium and posterior to the 
optic vesicle similar mesodermic proliferations from the ectoderm are 
found giving rise also in these regions to dense masses of cells with 
round nuclei — the Anlage of cartilage. The proliferation into the meso- 
derm is especially active about the mouth, as, owing to the cranial 
flexure several lines of proliferation appear to meet in the angle where 
the mouth breaks through, and the formation of the trabecular car- 
tilages may readily be followed. 
As yet my study has not extended to the formation of the car- 
tilages of the trunk, but homogeneousness of structure furnishes at 
least a presumption in favor of similarity of origin, otherwise one 
might be forced to doubt a fundamental difference between ectoderm 
and endoderm. 
While the study of Elasmobranch, Teleost and Chick leads me to 
believe that like observations may be made in these forms, I know 
