iSSg.] Segmentation of the Ovum. 469 
quite alike, and each of them produces two cells which are also 
very similar to one another ; then comes a division of the 
four cells into eight, four of which resemble one another and 
differ from the remaining cells which are also similar among 
themselves. Four of the cells are derived chiefly from the 
substance of the animal pole of the ovum and are very pro- 
toplasmic ; and the other four cells are constituted out of the 
substance of the vegetable pole and accordingly contain most 
of the deutoplasm of the ovum. The eight cells form an ir- 
regular spheroid, in the centre of which there is a space be- 
tween the cells ; this space is known as the segmentation 
cavity. 
The four cells of the animal pole progress in their divisions 
more rapidly than the four of the vegetable pole, but the latter 
when the yolk matter is at a minimum, as, for instance, in echi- 
noderms, do not lag much. From their unequal rates of divis- 
ion the two sets of cells come to differ more and more in size, 
those of the animal pole being much the smaller. The division 
of the cells take place so that the cells form a continuous layer 
of epithelium, one cell thick, stretching around the enlarged 
central segmentation cavity, (Fig. 2) and, the latter being 
an outside view of an Amphioxus blastula, cf. infra ; the 
epithelium consists of a larger area of the small cells of the 
animal pole, and a small area of the large 
cells of the vegetable pole. This stage of 
segmentation is known as the blastula 
stage ; the small cells are destined to form 
the ectoderm of the embryo; the large 
cells the entoderm; the central space is 
the segmentation- cavity ; the line along 
/En which the two parts of the epithelium 
(ectoderm and entoderm) join is known 
as the ectental line. 
Vertebrate type of segmentation. In the 
! vertebrates we find that segmentation also 
^g^enta- results in two epithelia, one ectoderm and 
^^^ ^^' one entoderm, joined at their edges, and 
