ver being devoid of such yolk (alecithal = without yolk),
necessitates a very early start in life, and, for reasons too
complicated to state fully here, the development in such a case is
considered particularly instructive and primitive by zoologists.

Section 14. The first thing to be seen in the developing cell is a
deepening circular groove (Figure 1, Sheet 21), which divides the
ovum into two parts. Another groove then cuts at right angles to this
subdividing the two into four (Figure 2). Another groove, at right angles
to both the former, follows, making the four eight (Figure 3). And so
subdivision goes on. The whole process is called segmentation or
cleavage.

Section 15. At the end of segmentation we get a hollow sphere of
small cells, the cells separating from one another centrally and
enclosing a cavity as the process proceeds. This is the
blastosphere, shown diagrammatically in Figure 4, and of which an
internal view, rather truer to the facts of the case as regards shape,
is given as Figure 5. The central cavity is the segmentation cavity
(s.c.).

Section 16. Invagination follows (Figure 6). In this process a portion
of the blastosphere wall is the tucked into the rest, as indicated by
the arrow, so that a two-layered sack is formed. The space ar. is the
archenteron, the primordial intestine, and its mouth is called, the
blastopore (bp.). The outer layer of this double-walled sac is called
the epiblast. For the present we will give the inner lining no special
term. The young amphioxus has, at this stage, which is called the
gastrula stage, a curious parallelism with such a lowly form as the
Hydra of our ditches. This latter creature, like the gastrula, consists
essentially of two layers of cells, an outer protective and sensory
layer, and an inner digestive one; it has a primordial intestine, or
archenteron, and its mouth is sometimes regarded as being a
blastopore. All animals that have little yolk, and start early in life for
themselves, pass through a gastrula stage, substantially the same
as this of amphioxus.

Section 17. The anus is perforated later near the region occupied at
this stage by the blastopore. Hence the anterior end of the future
amphioxus, the head end, is pointing towards the Figure 6, and the
letters ep. are marked on the side which will be dorsal.

Section 18. Figure 7 i. is a dorsal view of the gastrula at a somewhat
later stage, and here indications of distinctly vert