MUSEUM OF COMPARATIVE ZOOLOGY. 109 
to the exterior, the invagination of the ectoderm (eden) 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. 2 t) 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-elefts do not lie in an exaetly transverso 
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 tendeney 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-clofts, 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 (eedrm.'), 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, Iig- 
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. "Phe vertical parallel 
lines 19, 20, 21, through Figure 16, indicate the direction and the relative 
