DEVELOPMENT OF PERIPATUS NOY2E-ZEALANDI2E. 289 
one half of its surface, at which time it is thickest in the centre 
and thins gradually towards its edges. 
7 . The epibolic growth of the blastoderm continues until 
only a very small space in the middle of the ventral face of 
the ovum is left uncovered. 
8. A proliferation of the nuclei behind the uncovered area 
in the middle line takes place, forming a (in transverse section) 
keel-shaped mass of nuclei extending along about the posterior 
half of the ovum. The nuclei at the sides of the space also 
proliferate. 
9. The protoplasm round the space becomes inflected, and so 
forms a blastopore. The proliferating mass of nuclei or primi- 
tive streak increases in amount. 
10. The blastopore increases in length considerably, and 
becomes more open. The primitive streak also becomes wider 
and deeper, and a groove — the primitive groove — appears along 
its centre. Beneath the primitive groove a small cavity filled 
with yolk and bounded by columnar nuclei, and apparently 
homologous with the polar area of P. capensis, appears. 
11. The blastopore and primitive streak and groove increase 
and become more marked. The polar area disappears. 
12. Up to this stage no trace of any cell-outlines is visible, 
but the protoplasm forms a syncitium, in which nuclei are 
irregularly scattered. At this point a large gap is present in 
my investigations. 
13. A layer of yolk is present outside the embryo. This 
peripheral yolk becomes gradually absorbed, and various 
changes are undergone by the embryo (descriptions of which 
are given in (4), vide summary, p. 230) until it reaches the 
stage at which the absorption is complete, when the appendages 
begin to appear, &c. 
14. The later development, i.e. after the appendages are 
formed, is similar to that of P. capensis, the only interesting 
point of difference being that the duct of the first somite opens 
to the exterior. 
