626 The American Naturalist. Uuly, 
cept that the ectoderm shows the primitive groove, Fig. 15, 
and is fused with the axial cord (Achsenstrang) of the mesoderm, 
so that in the axial region the three layers are united. In the 
posterior region of the primitive groove the connection of the 
mesoderm with the inner germ layer is dissolved, but the connec- 
tion with the entoderm is retained. Behind the primitive groove 
the mesoderm extends, but lies free between the ectoderm and en- 
toderm. To recapitulate:—There is a long axial mesodermic 
thickening, which has the primitive groove over its posterior two- 
thirds ; the thickening in front of the groove is united with the 
entoderm and is called the head process; the thickening under 
the front half of the groove is united with both the ectoderm and 
entoderm ; the thickening under the hind half of the groove is 
united only with the entoderm, it is to be remembered that the 
mesoderm arises from the entoderm, and its connection with the 
ectoderm is, it seems to me, to be considered secondary. 
A comparative examination of the primitive streak, as described 
for various classes in the preceding pages, shows that it has a re- 
markable uniformity of organization. In alltypes it consists of 
an axial accumulation of mesodermic cells; this mesodermal 
axial cord overlies the archenteron, and sends out mesodermic 
tissue in a widening sheet between the ectoderm and entoderm, 
headwards, sideways, and backwards; the mesoderm is thickest 
posteriorly, z. e., in the region of the blastopore; when it first ap- 
pears it is intimately connected with the entoderm except in the 
neighborhood of the blastopore, where both entoderm and meso- 
derm unite with ectoderm. The mesoderm constitutes an axial 
mass, and offers no trace of a bilateral division or origin. Such 
a division is produced secondarily by the meeting of the medul- 
lary and notochordal grooves. The ectoderm in the region of 
the primitive streak consists of high cylinder cells, but it gradu- 
ally thins out toward the embryonic rim. The entoderm on the 
dorsal side of the archenteron consists of discrete cells, which 
soon acquire a distinctly epithelial arrangement; laterally and in 
front it passes over into the yolk, which may be cellular or a 
multinucleate mass. These features recur in all the types we 
have studied, though the variations are very great. In the imme- 
