296 Proceedings of Royal Society of Edinburgh. [april 2, 
One of tlie most important and earliest processes in the develop- 
ment of the vertebrate germ is the longitudinal inflection of the axis. 
The dorsal line is usually convex in the cephalic part and concave 
in the succeeding part of the trunk. The formation of the optic 
vesicles and of the different cerebral segments depends on the 
curves of the medullary tube, and by the imitation of these in a 
curve of india-rubber we can produce a similar series of enlarge- 
ments and contractions. Amphioxus is the only vertebrate which 
shows during its embryonic life a dorsal concavity of the cephalic 
end, and also the complete absence of optic vesicles and of cerebral 
segmentation. 
The curve of the axis in the head determines the relative posi- 
tion of the forehead, the mouth, and the pericardial cavity. When 
three parallel tubes undergo the same inflection with dorsal con- 
vexity, the end of the inferior will be behind that of the middle 
and superior tubes. The pericardial cavity will be farther back 
than the mouth, the mouth than the forehead. In the embryo of 
the Amphioxus the relation is inverted ; Amphioxus would have 
its mouth in front of the anterior end of the medullary tube, if the 
mouth opened at the end of the intestinal canal as in other verte- 
brates. But this is not the case; the anterior end of the intestinal 
tube forms two peculiar organs, and the mouth opens farther back 
on the left side of the body. 
Another example of the consequences of the inflections of the 
axis may be given by the history of the heart and of the neck in 
the higher vertebrates. The greater part of the heart belongs pri- 
mitively to the head, and the anterior end of the heart reaches the 
mandibular wall of the mouth. In earlier embryos it therefore 
forms a voluminous appendage to the hinder part of the head. But 
as the body is strongly curved in its farther development, the head 
and the tail are bent so as to meet. By this bending the heart is 
placed in the angle between the head and the chest. The head 
afterwards rises, and the heart remains in its secondary position. 
During this time the neck of the embryo is formed behind the 
angle of inflection as a cuneiform piece of the body, containing 
vertebrae in its dorsal, but no cavity in its ventral part. Its forma- 
tion depends also on the temporary bending of the head ; and in 
the lower vertebrates, as in fishes, where the whole process of total 
