196 ADAM SEDGWICK. 
Figs. 22 a—c, represent aseries similar to the above through 
the polar area of a rather older embryo. The front part of 
the polar area has a well-marked groove (Pl. XIII, fig. 22 a) 
which is the primitive groove. 
FoRMATION OF THE MESODERM. 
The nuclei of the mesoderm are derived from the nuclei of 
the polar area. The latter increase largely in number (PI. 
XIV, fig. 25 6) and form a primitive streak. An early stage 
of this process is shown in figs. 22a, 6. It begins at the 
front end of the area, but soon the nuclei of the whole area 
are implicated. They are constantly met with in a state of 
division. 
In the next stage, stage a, figured in Part 1, fig. 22, a well- 
marked primitive streak is visible when the embryo is examined 
from the surface. 
A series of sections through such an embryo (PI. XIV, figs. 26 
a—d) show that the blastopore is still traversed by a reticulum 
(figs. 26 a, 6), and that the primitive streak is largely developed 
(figs. 26 c, d), and its front part traversed by a well-marked 
groove. In the deeper parts of the primitive streak, at about 
the middle of its length, there is an area of protoplasm con- 
taining two (perhaps more) nuclei, and characterised by the 
relative predominence of the extra-nuclear protoplasm. This 
area is shown in section in fig. 26 d. I cannot help thinking 
that it is derived directly from the hinder part of the polar 
area of the previous stage figured in Pl. XIII, fig. 21, and 
Pl. XIV, fig. 24 d. It seems to me that while the nuclei of 
the polar area on each side of this structure constantly undergo 
division (fig. 22 c, 24d) the nuclei in this structure do not 
divide, but that it becomes overgrown ventrally by the proli- 
ferating lateral nuclei of the polar area (Pl. XIII, fig. 21), and 
thus comes to acquire a deeper position. ‘This would seem to 
imply that the growth of the mesodermal nuclei in the hinder 
part of the polar area is a bilateral process, that the cells on 
each side of the middle line only proliferate ; and I think that. 
a careful examination of the anterior part of the polar area 
