THE FOEMATIOX OP THE LAYERS IN AMPHIOXUS. 343 
Fig. 4. — Median sagittal section through an embryo in which the 
difference in staining quality between ectodermal and endodermal 
nuclei is beginning to appear. 
Figs. 5 a and h . — Two sagittal sections through an embryo in which 
the nerve-plate (n.p.) is just beginning to be recognisable. Fig. 5 a is 
a median section, and in it the rounded character of the cells in the 
neighbourhood of the growing point x is remarkable. Fig. 5 is a more 
lateral section (four sections to the right) in which the neighbourhood 
of X is composed of quiescent cells. 
Fig. 6. — A median section through a gastrula in which the archenteron 
has become hemisi^herical. 
PLATE 19. 
Figs. 7 a and b . — Two sagittal sections through an embryo in which 
the pi'ocess of elongation has begun. Fig. 7 o is a median sagittal section 
and fig. 7 h a moi-e lateral section. The long axis of the nerve-plate is 
at right angles to the diameter of the wide blastopore. Magnification 
fiOO diameters. 
Fig. 8. — Median sagittal section through a gastrula in which the 
blastopore is in process of being closed through the activity of the 
growth-centre at y. ch. The layer of cells which will form the noto- 
chord and which is already beginning to lose its yolk-granules. Magni- 
fication fiOO diameters. 
Fig. 9. — Horizontal section through an almost complete gastrula 
taken near the middle of the series. Magnification 600 diameters. 
Fig. 10. — Horizontal section through an almost complete gastrula 
taken near the upper and anterior lip of the blastopore. Magnification 
1)00 diametei’s. 
Figs. 11 n-e . — Five transverse sections through a just complete 
gastrula in which the first traces of the body-cavities can be seen. In 
fig. 11 a, the most anterior section, the collar-cavities (coU.) alone are 
visible. In fig. 11 both collar-cavities and trunk-cavities are seen. In 
fig. 11c only trunk-cavities are visible. In fig. 11 d the anal divei-ticulum 
(«.) is seen still connected with the more dorsal part of the archenteron 
(«'.) by a narrow canal. In fig. 11 e the anal divei-ticulum is shut off from 
the rest of the archenteron. ne. Neurenteric canal. Magnification 675 
diameters. 
PLATE 20. 
Figs. 12 a-d . — Four transverse sections through a larva in which two 
somites have been cut off from the trunk-cavity. In fig. 12 a the 
openings of the collar-cavities are seen on both sides. In fig. 12 5 
the collar-cavity is closed on the left side and lies on the outer side of 
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