116 BULLETIN OF THE 
ers, pass ventrally under the chorda. Here too, as in case of Figure 
25, are nuclei (v/.ff) the origin of which is doubtful. This condition 
obtains through the third protovertebra also. In the region from the 
third to the sixth protovertebra (Fig. 29) the condition of the meso- 
derm is quite different. Here the proximal part of the lateral layers is 
cut off, and constitutes the intermediate cell-mass (cl-mo.i’m.!, Figs. 28 
and 29). From the ventral and inner region of this structure cells are 
given off, which pass toward the median line. 
The intermediate cell-mass moves still farther medianward under 
the protovertebra, and the cells from the two sides accumulate in the 
median line under the chorda. Back of about the sixth or seventh 
protovertebra, the elements which have thus been aggregated under the 
chorda form a mass of considerable volume (cl-mo.i’m.', Plate V. Figs. 
36 and 37). In later stages this mass extends forward, but in a much 
reduced volume, into the region of the nephrostome, or third protoverte- 
bra (Fig. 33). 
The mesodermal elements which may be said to migrate toward the 
sabnotochordal region are the following : — 
(1) The original intermediate cell-mass in the region of its primary 
formation. 
(2) Individual cells from the intermediary cell-mass in the region of 
its secondary formation. 
(3) Individual cells from the lateral layers in the region of the neph- 
rostome, and forward to the head-mesoderm. 
(4) Individual cells from the ventral region of the protovertebra, at 
an early stage. 
(5) The formative tissue (Ziegler’s Bildungsgewebe) from the lower 
margins of the protovertebre at a much later stage. 
(c) Concerning the Fate of the “ Intermediate Cell-mass.”? —I have not 
made a detailed study of the fate of this structure in Fundulus, and my 
only reason for including a brief account of my observations on it is 
based upon the fact that some recent observers have held that it is 
intimately related to the beginning (Anlage) of the pectoral fins ; in 
fact, that it constitutes one of the sources of the mesodermal elements 
which are contributed to the latter structure. It seems to me that in 
Fundulus such a relation between intermediate cell-mass and pectoral 
fin is impossible ; the basis for this belief I shall point out later. 
It is very evident, however, that the lateral region of the intermediate 
cell-mass gives rise to the segmental duct. The anterior region of the 
segmental duct is formed at nearly the same time as its nephrostome. 
