110 . BULLETIN OF THE 
positions of the planes of the sagittal sections shown in Figures 19-21. 
The relation between the head-mesoderm and the splanchnopleure in 
the pectoral region is represented in Figure 19. This section is taken 
from a region too far from the axis of the embryo to show any trace of 
the protovertebre, nor is the fourth gill-pocket yet developed far enough 
in a lateral direction to reach this vertical plane ; but the auditory vesi- 
cle (vs. au., Fig. 19), which extends farther laterally (compare Fig. 19 
with Figs. 16 and 24), is shown in the section. Hence the head-meso- 
derm (Fig. 19, ms’drm.), as represented in this section, may be said to 
extend back of the auditory vesicle and laterally of the fourth gill- 
pocket, and to converge at its posterior margin into the single-cell layer 
(so’plu.) of the somatopleure. The ventral surface of the head-mesoderm 
may be regarded morphologically equivalent to the somatopleure. 
In Figure 20, drawn from a section about 223 » nearer the axis of the 
embryo than Figure 19, the fourth gill-pocket is intersected, and also 
the distal face of the first protovertebra. Here the head-mesoderm is 
represented by a voluminous mass of cells which abut upon the anterior 
boundary of the first protovertebra (pr’vr. 2). In the next section 
(Fig. 21), whose plane lies still nearer the axis of the embryo, the head- 
mesoderm (ms’drm.) passes back and fuses with the first protovertebra, 
no boundary between the two being distinguishable. From this double 
series of sections the close relation existing between head-mesoderm, 
protovertebre, and somatopleure must be evident. 
I shall now consider the changes taking place in the two parietal lay- 
ers during slightly older stages ; these present three principal features : 
(1) their shifting in a median direction along the ventral surface of the 
protovertebre ; (2) the contribution of elements from their proximal 
region ; and (3) the modification of the somatopleure in the formation 
of the beginning of the pectoral fin. But these changes, as in case of 
those involving the head-mesoderm, are taking place at the same time, 
and hence must be traced conjointly. The first two processes may be 
considered under the head of “ intermediate cell-mass.” 
V. INTERMEDIATE CELL-MASS. 
(a.) Brief Review of the Literature. — The term “intermediate cell- 
mass” (Mittelplatte) was originally applied by Waldeyer (’69) to the 
mass of mesodermal cells between the protovertebree and lateral plates 
in the Chick. The intermediate cell-mass in Birds is so well known that 
it is unnecessary to review its morphology in detail ; suffice to say that 
