
538 THE AMERICAN NATURALIST. | [Vor. XXXVI. 
separated on the inner surface, but not very sharply if at all, 
from the dorsal and dorsal-lateral walls of the archenteron." 
In the egg of Bufo lentiginosus, when the blastopore first 
appears as a slight depression among the yolk cells just below 
the equator of the egg (Fig. 9), the cells which form the dorsal 
wall of the segmentation cavity are all small, somewhat angular 
cells which contain a considerable amount of pigment, particu- 
larly the cells which compose the outer surface of the egg. 
Near the equator the cells are found to be somewhat larger 
and to contain much less pigment, while in the lower hemi- 
sphere the cells are-very large, and they have but a few scat- 
tered pigment granules. Up to this stage of development 
there is no visible separation of the cells into ectoderm, endo- 
derm, and mesoderm. The smaller cells resulting from rapid 
cell division are found in the upper part of the egg and the 
larger cells, which divide less frequently, are grouped around the 
lower pole. The mass of cells marked 77 in Fig. 9 undoubt- 
edly represent the région of the egg from which the mesoderm 
is formed. These cells are in no wise distinctive at this period, 
and their later development into mesoderm I consider to be due 
solely to their position in the egg during the formation and 
closure of the blastopore. The cells which are to become 
mesoderm form a layer around the egg at the equatorial region 
just inside the cells that are to become the ectoderm, as found 
to be the case in the frog's egg, according to Morgan. These 
mesoderm cells have many characteristics in common with 
the large yolk cells into which they grade, being larger, more 
rounded, and containing much less pigment than the small cells 
of the upper hemisphere. It seems probable, therefore, that 
they were originally produced by divisions of the yolk cells. 
When the lips of the blastopore have extended so as to form 
a crescent in surface view (Fig. 3) a frontal section through an 
egg in the region of the blastopore (Fig. 14) shows that the 
dorsal wall of the archenteron is formed of several layers of 
small cells which have absolutely no distinction between them 
except that the cells of the outer layer, which are more regu- 
larly arranged, contain a much greater amount of pigment than 
the other cells. At the sides of the archenteron the innermost 
