MUSEUM OF COMPARATIVE ZOOLOGY. 109 
to the exterior, the invagination of the ectoderm (ec’drm.') having almost 
reached the endoderm ; but the fourth gill-cleft (brs. brn.) has not yet 
advanced so far, as is shown by a section (Fig. 24) from the same embryo, 
but 30 farther back ; hence the head-mesoderm is now more voluminous 
in this region than in the region of the third gill-cleft, and more than it 
will be here in later stages. It is evident that, as the fourth cleft de- 
velops, it will invade the region now occupied by this mass of head-meso- 
derm, hence the latter must migrate. But since the gill-clefts develop 
from before backward, and since the third cleft has already so far devel- 
oped as to occupy together with the auditory vesicle practically all the 
space (Plate IV. Fig. 23), there is no possibility of that part of the head- 
mesoderm which is now back of the third cleft migrating or being pressed 
forward in order to yield its space to the intruding fourth gill-cleft.. Be- 
ing thus guarded in front and on its ventral boundary by the endoderm, 
on its axial boundary by the spinal cord, and on its lateral by the pericar- 
dial cavity, the only possible course left for this mesoderm is to pass in a 
posterior direction toward the pectoral region. Furthermore, the endoder- 
mal evaginations forming the gill-clefts do not lie in an exactly transverse 
dorso-ventral plane, but extend obliquely from below upward and _ back- 
ward as well as outward and backward; hence, as this evagination pro- 
ceeds, there is a natural tendency to press the head-mesoderm in advance 
of it in the same direction. It may be safe, therefore, to base upon these 
facts the inference that the position of the gill-clefts, and the order and 
direction of their development, tend to cause the head-mesoderm of the 
branchial region to pass in a posterior and lateral direction. 
The relation of the head-mesoderm to the axial mesoderm in the pec- 
toral region may be better understood by sectioning different parts of a 
single embryo in planes perpendicular to each other. An embryo in 
which the distal tip of the fourth gill-cleft (Plate III. Fig. 16, brs. brn.) 
extends about midway through the region of the head-mesoderm (ms’drm. 
cap.) toward the ectodermic invagination (ec’drm.’!), is sectioned trans- 
versely through the branchial region from the anterior end of the em- 
bryo to the plane of the fourth cleft, represented by the drawing, Fig- 
ure 16. The remaining part of the embryo is then re-oriented upon the 
microtome, and sectioned in a longitudinal (sagittal) direction, begin- 
ning on the left side of the embryo and proceeding toward the axis. 
Figure 16 represents the anterior face of the last section of the trans- 
verse series from an embryo treated in this way, and Figures 19-21 show 
the series of sagittal sections in the same embryo. The vertical parallel 
lines 19, 20, 21, through Figure 16, indicate the direction and the relative 
