THE FORMATIOX OF THE LAYERS IX AMPHIOXUS. 297 
move cylindvical, due iu all probability to a widely distributed 
increase of cells both in ectodertn and endoderm. This is the 
beginning of that persistent growth in length which 
converts the short plump embryo into the comparatively long 
attenuated two-da}"-old larva. The axis of the nerve-plate 
is clearly at right angles to the diameter of the blastopore, 
and how that can be reconciled with a theory of concrescence 
of the dorso-lateral lips of that aperture I leave it to advo- 
cates of that theory to settle. 
The last stage in the gastrulation with which I shall deal 
is represented in fig. 8. Here we see the process of closing, 
or more strictly speaking of narrowing the blastopore. A 
centre of groAvth and multiplication of cells has 
now appeared in the v e n t !• a 1 lip of the b 1 a s t o }) o r e, 
and in consequence this lip has grown upwards and pro- 
trudes as a rounded mass blocking the cavity of the blasto- 
pore. We are forcibly reminded of the yolk-plug in 
Amphibian embryos ; and at the same time we note that all 
the cells with vesicular nuclei are being brought within the 
confines of the archenteron. It was the discovery of this 
new centre of growth that led Cerfontaine to imagine that in 
the later stages of gastrulation ectodermal cells were being 
inflected at the ventral lip of the blastopore; it will be seen 
also that Klaatsch was perfectly justified in supposing that 
cells passed iu at this lip, only the cells that passed in are 
endoderm, not ectoderm. 
'I’he remnant of the blastopore is eventually found as a 
small pore at the posterior end of the nerve-plate, iu which 
position it becomes covered in by the meeting above it of the 
ectodennal folds which arise at the sides of this plate. 
Figs. 9 and 10 represent sections from series cut through 
nearly complete gastrulae in a horizontal direction, in order 
that some account may be taken of the condition of the 
“lateral” lips of the blastopore. Fig. 9 is a section from the 
middle of the series cutting the blastopore near the mid- 
horizontal line, whilst fig. 10 is a section which cuts the 
blastopore near the doi'sal lip. In both we see the sharp 
