BLASTOPORE, MESODERM AND METAMERIC SEGMENTATION. 27 
EXPLANATION OF PLATE II, 
Illustrating Mr. W. H. Caldwell’s paper on “ Blastopore, Meso- 
derm and Metameric Segmentation.” 
List of Reference Letters. 
Blastopore, bl. Mouth, ra. Anus, a. Primitive streak, p. s. Primitive 
groove, p. g. Posterior pit, g. Posterior solid cord, r. Anterior mesodermic 
diverticulum, a. d. Posterior mesodermic diverticulum, p. d. Mesoderm, me. 
Prseoral lobe, p. 1. Arckenteron, ar. External opening of nepkridium, n. 
Body cavity (coelom), c. Splanclmopleuric vascular space, v. Nutritive (?) 
body, x. 
All the figures, both of whole embryos and of sections, were drawn by 
means of Zeiss’s two-prism camera, from permanent preparations in Canada 
balsam. The embryos were treated as follows : — Mixture of two volumes 
corrosive sublimate 4- one vol. glacial acetic acid for 1 second, water 15 
seconds, alcohol, 50 per cent., 5 minutes, alcohol 70 per cent. 
Pigs. 1 — 7. Zeiss’s oc. 2, obj. D. Pigs. 8 — 18. Zeiss’s oc. 2, obj. l-12th 
homog. imm. All the embryos belong to the species of Phoronis, living in 
the harbour of Naples in dense colonies without sand adhering to their tubes, 
except that of which Fig. 13 is a section. This is an Australian Phoronis, 
discovered in Port Jackson by Mr. Haswell, of Sydney University. The 
sections drawn are taken from complete series. Each section is '005 mm. 
thick. I have series ’0025 mm. thick. These, though necessary for observa- 
tion purposes, are not so convenient for drawing. 
Figs. 1 — 4. — Pour embryos, showing mode of closure of the blastopore. 
Fig. 1. Blastopore, bl., before meeting of lips. 
Pig. 2. Blastopore fused posteriorly : open part=mouth, m. ; closed part= 
primitive streak and groove, p. g. 
Fig. 3. Primitive groove, p.g.; disappearing prseoral lobe, p. 1. 
Fig. 4. Primitive groove enlarging into pit, g., posteriorly; prseoral 
lobe, p. 1. 
Figs. 5 and 6. — Two embryos viewed from the left side. Prseoral lobe, 
p. 1 . ; primitive groove, p. g. ; mouth, m. ; posterior pit, g. 
Fig. 7. — Older embryo with complete alimentary canal. Mouth, m . ; anus, 
a. ; external opening of nephridium, n. 
Figs. 8 — 10. — Three sections in a nearly transverse plane of an embryo in 
a stage between that of Fig. 1 and that of Fig. 2. 
