DEVELOPMENT OF THE CAPE SPECIES OF PERIPATUS. 197 
shows that the growth of nuclei there also is a bilateral one, 
though the bilateral nature of the growth is not so obvious as 
it is behind. The reason of this is that behind there is a 
median structure—the hinder part of the polar area with its 
round nuclei—on each side of which the growth appears to 
take place, while in front there is no such well-marked 
median structure, but there is the groove; and I think that a 
careful examination of the relation of the growing nuclei to 
this groove shows the bilateral nature of the growth. I refer 
in support of this to figs. 22 a, 25 6, 26c, which are all 
sections through the front part of the primitive streak, fig. 22 a, 
being of course from the youngest of the embryos; and to 
figs. 25 a, 26 6, which are in each case the last section through 
the blastopore. It is difficult to say whether 25 a is to be 
regarded as passing through the hind end of the blastopore or 
through the front end of the streak, and in this figure there 
are nuclei, which must be regarded as mesodermal, placed in a 
position which looks very much as though they were derived 
from the row of nuclei which extend between the ectodermal 
and endodermal nuclei. 
Again, in fig. 26 6, we see similarly placed nuclei in the 
act of division, with what must be regarded as mesodermal 
nuclei on their inner borders. 
Further back (figs. 22 a, 25 6, 26 5) the blastopore is re- 
presented only by the groove, and it is more difficult to satisfy 
oneself on the point. 
However, I am inclined to think that the growth of primitive 
streak nuclei is a bilateral one, in the anterior as well as in the 
posterior part of the primitive streak, though I admit that the 
evidence in favour of this view is not entirely satisfactory. 
If I am correct in this supposition, and in my conjecture 
that the primitive groove is a rudimentary posterior part of the 
blastopore (it is so considered in other tracheate embryos), 
then the development of the mesoderm in Peripatus consists in 
an ingrowth of mesoderm from the lips of the blastopore and 
resembles that described in so many other forms, 
The mesodermal nuclei of the primitive streak now grow 
