THE DEVELOPMENT OF OPHIOTHRIX FliAGILIS. 603 



I'iG. 19. — Longitudinal section of a blastula. Artificial fertilisation, mes. 

 Primary niesencliyme filling the interior. X 500. 



Fig. 20. — Longitndinal section of a blastula in wliicli gastrulation is about, 

 to commence. Artificial fertilisation, mes^. Primary mesenchyme filling the 

 interior, mes^. Secondary mesenchyme budded from the endodermic area. 

 X 500. 



Fig. 21. — Longitudinal sagittal section of a gastrula. Artificial fertilisa- 

 tion. Notice the tongue of endodermic cells projecting into the archentcron. 

 X 500. 



PLATE 33. 



Fig. 22. — Longitudinal frontal section of a larva two days old. Artificial 

 fertilisation, p. I. First rudiment of one of the postero-latcral arms, x 500. 



Fig. 23. — Longitudinal frontal section of a larva two and a half days old, 

 Artificial fertilisation, ccc. Ccelomic rudiment still opening into the archen- 

 tcron. X 500. 



Fig. 21. — Longitudinal section of blastula seven hours old. Natural fertilisa- 

 tion, mes. Beginning mesenchyme. X 500. 



Fig. 25. — Longitudinal frontal section of incipient gastrula, one d.iy old. 

 Natural fertilisation. X 500. 



Fig. 20. — Longitudinal frontal section of more advanced gastrula, one day 

 old. Natural fertilisation. X 500. 



Fig. 27. — Longitudinal frontal section of gastrula of one and a half days. 

 Natural fertilisation. X 500, 



Fig. 28. — Longitudinal frontal section of larva, two days old. Natural 

 fertilisation. Notice the origin of the coelom. car. Ccelomic rudiment, p. I. 

 Rudiment of the postero-lateral arm. X 500. 



Fig. 29. — Longitudinal frontal section of larva, two days old. Natural 

 fertilisation. Notice that the coelom is separated from the archentcron as a 

 single vesicle. X 500. 



Fig. 30. — Longitudinal frontal section of a larva two and a half days old. 

 Natural fertilisation. The stomodaum is in the act of opening into the oeso- 

 phagus. oes. ffisophagus. stom. Stomodajum, X 500. 



Fig. 31. — Longitudinal frontal section of a larva picked out of Plankton. 

 Estimated age three and a half days. muse. Muscular fibres encircling oeso- 

 l)hagus developed from ccelomic sacs, x 200. 



