106 BULLETIN OF THE 
laterally than the somatopleure, and the margins of both lying close 
against the ectoderm. 
The axial mesoderm, or protovertebral part (ms'drm.), consists in this 
region of only a few cells. These lie between the proximal portion of 
the lateral layers and the brain (en’c.), and are bounded dorsally by | 
the incipient auditory vesicle (vs. au!) and ventrally by the endoderm 
(en’drm.). The axial mesoderm may be traced as head-mesoderm from 
this point caudad into a region where it is more or less distinctly seg- 
mented into protovertebr& (Figs. 12-14). 
In the middle region of the auditory vesicle (Fig. 10), the axial 
mesoderm becomes more voluminous and presses the pericardial cavity 
farther away from the axis of the embryo. The lateral layers are not 
separated from the axial mesoderm (compare Fig. 9), and their distal 
margins are united. The most anterior protovertebra in this stage 
(Fig. 11) is not wholly cut off from the lateral mesodermal layers. The 
lumen of the body cavity (cel.) can be traced cephalad into the pericar- 
dial cavity, and extends into the interior of the protovertebra as well as 
laterally between the somatopleurie and splanchnopleuric layers. The 
dorsal boundary of this lumen is a single-cell layer, while the layer on its 
ventral side is of variable thickness; in its proximal or protovertebral 
region it is about three cells in thickness, while in the middle it is two 
cells and at its distal margin only one cell thick, thus giving in cross 
section a cuneiform outline; it represents a portion of the protovertebra 
and the undifferentiated splanchnopleure. The section drawn in Fig- 
ure 12 is the third section (about 224 p) back of Figure 11, and represents 
the posterior part of the corresponding (first) protovertebra on the 
opposite side of the embryo.’ In this region of the protovertebra, the 
dorsal contour is more rounded, and the body cavity does not extend 
into it; the nuclei outline a boundary between the protovertebra and 
the lateral plates more distinctly than in the anterior region of the pro- 
tovertebra (compare Fig. 11). 
The second protovertebra is cut off from the surrounding mesoderm 
more completely than the first. A cross section from its middle region 
(Fig. 13) shows a more or less distinct boundary separating it from the 
lateral layers. It has a sharply rounded contour at its dorsal limit, and 
the nuclei show a tendency to assume a peripheral position. The body 
1 The embryo being cut exactly in the transverse plane, and the conditions in the 
mesoderm on both sides of the axis being identical, I have taken my drawings 
alternately from the right and left sides (Plate IL), in order to economize space in 
the arrangement of the figures on the Plate. 
