MUSEUM OF COMPARATIVE ZOOLOGY. 11:5 
mental duct it remains here for a time as an undifferentiated mass of 
cells, from the ventral part of which individual cells sever their connec- 
tion and pass to a position under the chorda. In the region of the 
nephrostome, where no intermediate cell-mass is formed, individual cells 
are given off from the proximal margin of the lateral layers, and pass 
ventrally under the chorda in a manner similar to those contributed by 
the intermediate cell-mass. 
In an embryo of about three days the intermediate cell-mass is dis- 
tinctly formed from the vicinity of the sixth protovertebra backward, 
but in this region (Plate II. Fig. 15, c/-mo.?m.") and through a short 
distance in front of it (Fig. 14%, c/-mo.?m."), although the protovertebrae 
are distinctly cut off from the remaining portions of the mesoderm, yet 
the line of separation between intermediate cell-mass and lateral layers 
is only faintly outlined. A little later, in an embryo of three and a half 
days, this structure is distinctly differentiated over a region extending 
from the sixth (Figs. 28, 29) as far forward as the third protovertebra 
(Fig. 26). 
At this stage the ventral migration of mesodermal elements begins in 
this region. At first only a limited number of cells migrate to the 
median line under the chorda (Plate IV. Fig. 27%, »/.*, n/.T), but later 
they are more numerous (Plate V. Fig. 33, ci-mo.?m.") ; yet the total 
number of cells which reach the median line is never as great as in 
the posterior part of the trunk. 
In the region of the second protovertebra (Plate IV. Fig. 25) the 
endoderm abuts bluntly against the proximal region of the lateral 
layers, which has shifted about half-way down along the side of the 
protovertebra. Here there is no positive evidence of any outgrowth 
from the lateral layers toward the median line, but frequently nuclei 
are found which are of doubtful origin (u/.fT, Fig. 25), arising either 
from protovertebra or from lateral layers. In the region about opposite 
the middle of the third protovertebra (Fig. 27) the endoderm has not 
yet begun the infolding of its distal margin to form the intestine, and 
hence it extends farther out under the lateral layers than in the region 
of the second protovertebra (compare en’drm., Figs. 25 and 27), and 
the lateral mesodermie layers (so’plu. and spl’plu., Fig. 27) extend far- 
ther under the protovertebra than they do in a more anterior region. 
In a later stage, the nephrostome is formed in the proximal region 
of the lateral layers, opposite this protovertebra (Fig. 27). In the 
same protovertebra, but in its posterior region (Fig. 27°) nuclei (»/.T) 
which evidently originate from the proximal region of the lateral lay- 
