15589- J Segmentation of the Ovnvi. 475 
rious directions. Not only are the small central segments divided 
into still smaller ones, (Fig. 5, D.) but their number is increased 
also by the addition of cells cleft off from the central ends of 
the large peripheral segments, which are themselves subdivided 
by additional radiating furrows (Fig. 5, D.). Sections of the 
hardened germinal disc show that segmentation is not confined 
to the surface, but extends through the protoplasmic mass of 
the animal pole, there being deep seated cleavage in planes 
parallel to the surface of the ovum. According to Duval, 19, 
when the first few, small central cells are separated off there is 
a small space between them and the underlying egg substance 
(see Figs. 2, 3, 4, 5 and 6 of his Plate I.) and this space he 
calls the segmentation cavity ; but in this, I think, he is in 
error, for the cells formed below this space are incorporated in 
the ectoderm or primitive blastoderm ; the cells referred to 
are those marked im, in Fig. 8 of Duval's PL I. The true seg- 
mentation cavity, as we have seen, is bounded on one side by 
entoderm. This fundamental characteristic Duval has en- 
tirely overlooked. From the processes described, there re- 
sults a disc of cells, which receives peripheral additions ; the 
border from which these additions come is known as the 
segmenting zone. The whole mass of cells derived from the 
germinal disc represents the ectoderm, and the segmenting 
^one may be homologized with the cells around the edge of 
the primitive blastoderm of the frog (Fig. 4, k w). A section 
through the segmented germinal disc shows the following 
relations : The blastoderm is a disc of cells ; its upper layer 
IS epithelioid, its lower layers consists of rounded cells more 
or less irregularly disposed ; at its edge it merges into the 
yolk which continues to produce cells ; between the blasto- 
derm and the yolk is a fissure— the segmentation cavity ; the 
yolk under the fissure contains a few nuclei, which have each 
a little protoplasm about them, but do not form parts of dis- 
crete cells. 
In reptiles, the process of segmentation is very similar to 
that in birds. Our knowledge is based principally upon obser- 
vations upon the eggs of the European lizards {Laccrta agilis 
and viridis) which have been studied by Kupfferand Benecke, 
