624 The Amerwan Naturalist. [July 
quently expands very far and acquires a more cellular character; 
this shelf is the commencement, therefore, of the Kezmwall of 
German writers. Immediately above the entoderm and intimate- 
ly connected with it are a few cells which belong to the meso- 
derm, C, Mes; the ectoderm is quite thick, C, Æc, and consists 
of high columnar cells; towards its periphery the ectoderm thins 
out, and its edge rests upon the yolk, with which it has no con- 
nection. In the region of the primitive streak, Fig. 14, 2, 
the fundamental relations are the same, but there are important 
differences to note. The entodermic cavity, Ach, is very much 
smaller ; the mesoderm is much thicker, and in the axial region 
fuses with both the ectoderm and entoderm, thus forming the 
Achsenstrang (axial cord) of German writers; the mesoderm also 
spreads out over the yolk far beyond the archenteric cavity, about 
one-third of the way from the axial line to the distal edge of the 
ectoderm; the ectoderm is disposed about as in the previous 
section, except that in the centre it merges into the mesoderm 
and presents externally a small notch corresponding to the section 
of the primitive groove. There appears to me no satisfactory 
evidence that the mesoderm receives, as some writers have main- 
tained, peripheral additions from the yolk. In both sections the 
yolk under the blastoderm contains numerous nuclei near its 
surface. 
Modifications very soon ensue in the Sauropsida (birds and rep- 
tiles) by which the disposition of the mesoderm is considerably 
changed, especially in three respects, namely: by the appearance 
of the so-called head process (Kopffortsatz), by the axial connec- 
tion of the mesoderm with the ectoderm, and by its losing in part 
its connection with the mesoderm. During these changes the 
archenteron expands rapidly, the archenteron expands rapidly, 
the entoderm becomes very thin in the region of the are apellucida, 
and passes more and more abruptly, as development progresses, 
into the so-called germinal wall of the area opaca; finally the 
ectoderm becomes thinner peripherally ; so that the axial thicker 
part is gradually marked off more and more abruptly. Sections 
of a stage with a primitive groove at its maximum,—a stage 
which is usually found towards the end of the first day of incu- 
