MUSEUM OF COMPARATIVE ZOOLOGY. 105 
caudal region, In an embryo of about fifty-six hours (Fig. 8), five 
protovertebree are distinctly formed, and the mesoderm back of the fifth 
protovertebra as far as the caudal region still consists of a compact plate 
of cells (ms’drm. aw.), whose boundary is easily distinguished from the 
ectoderm above and the endoderm below. The nuclei in the region of 
the axial mesoderm are undergoing rapid division as shown by the 
abundance of their various karyokinetio conditions. The protovertebra 
which are already formed in an embryo at the beginning of the third 
day, are somewhat irregular in outline and vary considerably in size, 
The first (Fig. 8, pr'vr. 1) is considerably larger in its antero-posterior 
dimensions than any one of the other four. This condition I have 
observed in several instances, but apparently it does not always obtain ; 
for in two embryos of the same age, but perhaps slightly more advanced 
than that of Figure 8, I find the first protovertebra not only shorter in 
its antero-posterior direction, but also decidedly more irregular in out- 
line than the others in the same embryo. Possibly the superior size of 
the first protovertebra in an earlier stage may be the rule, but this 
condition must be very transitory ; so that, owing to individual variation, 
embryos of the same age, but slightly more advanced, may show the in- 
ferior size and imperfect contour in the first protovertebra as a secondary 
condition, The axial mesoderm in front of the first protovertebra (Fig. 8) 
extends forward under the auditory vesicle, and its cells are disposed in 
two more or less distinct layers, the boundary between which extends 
outward, i. e. laterally from the axis of the embryo, where it is con- 
tinuous with a similar boundary (Fig. 7, cod.) between the layers of the 
lateral plate in the trunk region. 
The changes in the development of the mesoderm in the pectoral 
region can be most profitably studied from transverse serial sections. 
The differentiation of the protovertebre and lateral layers, and the for- 
mation of the eclomie split, are intimately involved, and must therefore 
be traced conjointly in the pectoral region. 
In the anterior region of the auditory vesicle, the axial mesoderm is 
reduced to its minimum volume. A transverse section through this 
region from an embryo of about seventy-one hours shows (Plate II. 
Fig. 9) that the mesoderm consists of two portions, the axial and the 
lateral; the latter is composed of two single-cell layers, somatopleure 
and splanchnopleure (so’plu., spPplu.), which in this region form the 
boundaries of the pericardial cavity (cav. prer.). These layers are con- 
tinuous with each other in their proximal (axial) region, but separate 
at their distal (lateral) margins, the splanchnopleure extending farther 
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