200 ADAM SEDGWICK. 
multinucleated mass of protoplasm, and the gastrula of 
Peripatus is a multinucleated mass or syncytium, with absolute 
continuity of the protoplasm of all parts of the ovum. 
The Mesoderm.—After the definite formation of the blastopore, 
an area of protoplasm, placed in the ectodermal layer of the 
syncytium, and characterised by possessing several nuclei less 
densely packed together than elsewhere, is distinctly visible in 
the middle line of the ventral surface just behind the blastopore. 
This area I have called the polar area. Its nuclei undergo 
division and give rise to the densely packed mass of nuclei of 
the primitive streak. A part of it seems to persist for some 
time in the deeper parts of the primitive streak close to the 
endoderm. 
The nuclei of the primitive streak migrate forwards between 
the ectodermal and endodermal nuclei, and take up their po- 
sition in the protoplasm intervening between the latter. 
These rows of nuclei are the mesodermal bands. They soon 
arrange themselves into groups around a central vacuole, and 
so give rise to the most conspicuous parts of the mesoblastic 
somites. I leave the ovum for the present at the commence- 
ment of the formation of the somites, merely stating that it is 
still a syncytium. 
There are a certain number of facts in the above account 
which are of general interest and seem to deserve more dis- 
cussion so far as their relation to processes in other forms are 
concerned. These are: 
1. The connection between the intra- and extra-nuclear re- 
ticulum. 
2. The segmentation. 
3. The origin of the gut as a vacuole. 
4, The syncytial nature of the embryo. 
5. The origin of the mesoderm. 
I propose to consider some of these points at once, and to 
defer the 5th, to Part 3 of this series. 
a. The nucleus of the unsegmented ovum and of the early 
stages of segmentation of the Cape Peripatus are particularly 
favorable for study, because of their large size and the rapid 
