MUSEUM OF COMPARATIVE ZOOLOGY. 103 
clove-oil preparation, and the chorda is formed as far forward as the 
pectoral region, where it is now closing in, but is not yet distinct from 
the endoderm (cd., Fig. 6). The endodermal cells in the parietal regions 
are rapidly moving in the direction of the embryonic axis, but no 
trace of the lateral infolding to form the tube of the intestine is as yet 
discernible. 
The condition of the mesoderm may be observed in a transverse sec- 
tion (ms’drm., Fig. 6) of an embryo perhaps a little more advanced than 
the one represented by Figure 2, taken from the region indicated by the 
line at 6 in Figure 2. In this region the mesoderm appears as a solid 
mass of cells which, in transverse section, has a triangular contour, 
ending in blunt angles at the dorsal and ventral limits of its axial 
boundary, and tapering laterally into a single-cell layer, which now 
extends beyond the lateral margin of the endoderm, the latter having 
shifted towards the axis of the embryo to form the chorda, as already 
noted. It follows, therefore, that the distal margins of the lateral 
mesodermal plates lie directly upon the yolk. The differentiation of the 
mesoderm into protovertebre and lateral plates has evidently not yet 
begun at this stage, and no trace of a split in the mesoderm to form 
the somatopleure and splanchnopleure can be seen.! 
The most anterior protovertebra is formed immediately back of the 
auditory vesicle ; in fact, it at first extends rather forward and below 
the posterior limit of the auditory vesicle (Fig. 8, pr’vr. 1). It is inter- 
esting to note that later a considerable space intervenes between the lat- 
ter and the most anterior protovertebra. (Compare Fig. 21, Plate III.) 
This intervening space is equal to the length of at least one proto- 
vertebra, and still later (Plate I. Fig. 8%) to about two protovertebre. 
There is some uncertainty as to the process by which the increase in 
this space is produced. There are at least four possible methods of 
increasing it: first, there might be an actual shifting of the anterior 
protovertebra caudad ; secondly, a shifting of the auditory vesicle ceph- 
alad is conceivable ; but I believe that neither of these is realized ; 
thirdly, there might be a relative shifting of these structures in the 
respective directions named, caused by a voluminous cell proliferation 
of the head-mesoderm (ms’drm. cap., Fig. 21, Plate III.) between the 
auditory vesicle and the first protovertebra, causing an elongation of 
the embryonic axis in this region ; fourthly, the first —and perhaps 
also the second — protovertebra may degenerate, remaining only as 
1 T have used the words somatopleure and splanchnopleure in the sense of somatic 
and splanchnic mesoderm respectively. 
