MUSEUM OF COMPARATIVE ZOOLOGY. 119 
in the anterior region of the trunk has just closed, it has again entirely 
disappeared. The identity of these nuclei is lost at the time of the 
formation of the dorsal aorta, but the subnotochordal rod, according 
to Balfour’s (’81, pp. 620-622) account, persists until later, and then 
atrophies, 
Whatever may be the source and fate of these cells in Fundulus, it is 
certain that there are other cells contributed from the ventral margins 
of the protovertebre which do not take this definite arrangement, and 
whose fate is not involved in much doubt. These nuclei (n/., Fig. 30, 
and nl.*, Figs. 24, 25) I believe to be concerned in the formation of the 
walls of the aorta. In later stages, subsequent to the formation of the 
aorta (Figs. 42-44), there is also a very large contribution of elements 
from the ventral region of the protovertebr to the median line. The 
greater part of the axial intermediate cell-mass is probably converted 
into blood corpuscles, which are seen enclosed within the delicate wall of 
the aorta (Figs. 35 and 43) from its earliest formation. 
By a comparison of Figures 34 and 35 (Plate V.) from the fifth somite, 
the change which takes place in the axial cell-mass may be appreciated. 
These drawings represent sections from the corresponding regions of 
embryos of four and five days respectively. In Figure 34, no differen- 
tiation of the intermediate cell-mass (cl-mo. ’m.’) is to be seen, but it 
consists of closely packed indifferent cells. In Figure 35, the aorta is 
formed, its walls being outlined by delicately drawn-out cells, in sec- 
tion spindle-shaped, enclosing blood corpuscles (cp. san.), which must 
have originated from the cell-mass. Outside the walls of the aorta, and 
between it and the intestinal canal, are seven nuclei, which are the 
remaining representatives of the original cell-mass. The change has 
resulted in the densely packed cell-mass being replaced by the aorta, its 
contained blood corpuscles, and the small number of nuclei on its ventral 
side where the trunk vein will be formed later. The aorta can be traced 
from the anterior trunk region to about the seventh somite. 
The number of nuclei, or blood corpuscles, lying within the aorta, 
increases in passing back from the region of the nephrostome, until a 
point is reached where it is relatively large, and the mass of cells within 
it seems to be continuous with the original cell-mass, and aortic walls can- 
not be readily distinguished. 
The following table shows the variation in the number of nuclei 
lying inside and those lying outside the aorta, through a series of con- 
secutive sections, beginning with the fifth somite of an embryo of about 
five days. 
