28 
W. H. CALDWELL. 
Fig. 8. In front of mouth. Anterior mesodermic diverticula, a. d . ; anterior 
mesodermic cell, me . ; archenteron, ar. 
Fig. 9. Through the fusing lips of the blastopore. Primitive streak, p. s. ; 
archenteron, ar. 
Fig 10. Through the posterior part of the primitive streak. Primitive 
groove, p. g . ; posterior solid cord of cells, r. 
Fig. 11. — Median section, longitudinal vertical, through a slightly older 
embryo than Figs. 8, 9, and 10. Pneoral lobe, p. 1 . ; anterior mesoderm, me . ; 
mouth, m . ; archenteron, ar . ; posterior pit, g ; posterior solid cord of cells, 
r ; Nutritive (?) body, x ; body cavity, c. 
Fig. 12. — Embryo slightly older than Fig. 4. Median longitudinal vertical 
section. Prseoral lobe, p. 1 . ; anterior mesoderm, m !. ; body cavity, c. ; vascu- 
lar space, v . ; middle mesoderm, m ". ; posterior mesoderm, m'". ; archen- 
teron, ar. ; posterior pit, g ; Mouth, m. 
Fig. 13. — Embryo, Australian species, nearly same stage as Fig. 12. 
Longitudinal horizontal section. Archenteron, ar . ; anterior mesoderm, me ! ; 
posterior mesodermic diverticula, p. d . ; posterior pit, g. 
Fig. 14. — Nearly same section as Fig. 13. Vascular space, v. ; posterior 
pit, g ; archenteron, ar . ; anterior mesoderm, me". ; posterior diverticula, 
p. d . ; posterior mesoderm, me'". 
Fig. 15. — Embryo, same stage as Fig. 12. Longitudinal horizontal section. 
Reopening of posterior cord of cells to form rectum and anus, a. ; anterior 
mesoderm, me ' . ; archenteron, ar. 
Fig. 16. — Embryo, same stage as Fig. 12. Transverse section through 
posterior diverticula, p. d . ; posterior mesoderm me '" . ; archenteron, ar. 
Figs. 17 and 18. — Embryos about the stage of Fig. 3. Two transverse 
sections. Anterior diverticula, a. d . ; body cavity, c . ; vascular space, v. ; 
primitive streak, p. s . ; primitive groove, p. g. ; middle mesoderm, me " . ; arch- 
enteron, ar . ; nutritive (?) body, x. 
Fig. 19. — Six diagrams, illustrating the typical modes of mesoderm forma- 
tion, e. g. 
1. Peripatus, cf. Hertwig’s “ Ccelom-theorie,” PI. ii, fig. of insect embryo. 
2. Amphioxus. 
3. Nemertine larva of Desor (?), Phoronis (posterior). 
4. Pristiurus, Phoronis (anterior). 
5. Lopadorhynchus (Kleinenberg). 
6. Primitive streak region, many Vertebrates and Phoronis. 
