DEVELOPMENT OF PERIPATUS NOV/E-ZEALANDDE. 285 
about one third of the length of the ovum, and having in 
transverse section an irregular triangular shape, the base of 
the triangle resting on the surface; nuclei are also present 
throughout the yolk, more especially near the periphery. 
As was said in (4) the protoplasmic area next becomes more 
compact and flattened out against the side of the ovum, so as 
to form a kind of plate of fairly dense protoplasm which is 
closely packed with nuclei, and which lies on the surface at one 
side of the ovum through about the middle third of its length : 
this is shown in transverse section in fig. 13 (4), which is from 
one of the ova taken out in April. 
A good many of the January eggs were of stages which 
came after this last. The protoplasmic area begins to grow 
round the surface of the egg until, as is shown in fig. 1, it 
covers nearly half the periphery. The nuclei in the central 
region of this protoplasmic area are two or three deep, but 
towards its edges the protoplasm thins gradually and the nuclei 
form only one layer. The ovum is closely packed with yolk, 
among which a few nuclei are irregularly scattered. The pro- 
cess of epibolic growth of the protoplasmic area continues, so 
that, as is shown in fig. 2, still more of the periphery of the 
egg becomes covered with the thin layer of protoplasm which 
may be called the blastoderm, until when the growth of the 
latter ceases only a very small portion is left uncovered. 
The blastoderm now consists of a thin layer of protoplasm, 
in which a single layer of flat nuclei is present, extending 
round the whole periphery of the egg with the exception of a 
small space lying in the centre of one side of the egg in which 
the yolk is left uncovered. This uncovered space corresponds 
with the position of the future blastopore, and is, I believe, 
situated on the ventral side of the embryo, thus agreeing with 
other Arthropods, except the Scorpions, in which the blasto- 
pore is situated dorsally. 
A change now takes place in the blastoderm behind and at 
the sides of the uncovered area. In the middle line behind 
this area a proliferation of the nuclei takes place extending 
some distance backwards, so that a keel-shaped mass of nuclei 
