86 HUBERT LYMAN CLARK ON 



PLATE 11. 



Fig. 1. Mature ovmn. 225x. 



Fig. 2. Two-cell stage of segmenting egg. 225x. 



Fig. 3. Four-cell stage. 225x. 



Fig. 4. Eight-cell stage. 225x. 



Fig. 5. Sixteen-cell stage. 225x. 



Fig. 6. Thirty-two-cell stage, seen from the side. 225x. 



Fig. 7. Thirty-two-cell stage, seen from one of the poles. 225x. 



Fig. 8. Blastula, seen from the side. 225x. 



Fig. 9. Gastrula, seen from the side. 225x. 



Fig. 10. Older gastrula, seen from left-hand side, to show formation of the water-pore. 



Fig. 11. Still older stage, seen from left-hand side, to show the drawing away of the archenteron from the water- 

 pore. 225x. 



Fig. 12. Slightly older stage, seen from left side, to show the formation of the mouth. 225x. 



Fig. 13. Older stage, seen from in front (ventrally), to show the formation of the enterocoel. 225x. 



Fig. 14. Older stage, seen from in front (veutrally), to show the formation of the coelomic vesicles. 225x. 



Fig. 15. Older stage, seen from in front (ventrally), to show the five primary outgrowths of the hydrocoel. 225x. 



Fig. 16. Older stage, seen from left side, to show the hydrocoel, body-cavities, and atrium. 225x. 



Fig. 17. Very young pentactula, from right side, to show the nerves and sense-organs. 225x. 



Fig. 18. Older pentactula, seen from dorsal surface, to show the radial nerves, calcareous ring, and rudiment of accessory 

 tentacle. 130x. 



Fig. 19. Ten-tentacled young, seen from right side, to show genital gland and arrangement of organs. Twelfth tentacle 

 just developing. 22x. 



Fig. 20. Adult Synapta vivipara, seen from ventral surface. Nat. size. 



PLATE 12. 



Fig. 21. Vertical section of gastrula, to show the thickened ectoderm at apical pole. 225x. 



Fig. 22. Transverse section of Fig. 13, at the line A. B., to show the thickened ventral ectoderm. 225x. 



Fig. 23. Schematic outline of hydrocoel, to show the position of water-canal. 



Fig. 24. Transverse sections of hydrocoels of three embryos, to show formation of anterior enterocoel. a, youngest 

 stage; b, somewhat older; c, oldest stage. 225x. In c, only a very small part of the hydrocoel is shown. 



Fig. 25. Transverse section of larva, somewhat older than Fig. 16, to show closure of hydrocoel ring, without the forma- 

 tion of a Polian vesicle. 225x. The section is very oblique, and takes in a part of the floor of the atrium (A), and only a 

 portion of the hydrocoel. 



Fig. 26. Transverse section of larva like Fig. 16, to show anterior prolongation of the left coeloin. 225x. 



Fig. 27. Transverse section of same larva, somewhat higher up, to show the left prolongations of the left coelom. 225x. 



Fig. 28. Longitudinal section of larva like Fig. 16, very badly preserved, to show the anterior prolongation of the left 

 coelom, ant. 1. c. 225x. 



Fig. 29. Posterior end of an adult, to show the rupture of the body-wall, caused by birth of the young. 35x. 



Fig. 30. Transverse section of the posterior end of an adult, to show one of the openings from the rectum into the body- 

 cavity. 65x. 



Fig. 81. One of these openings more highly magnified. 500x. 



Fig. 32. Part of a transverse section of a young pentactula, to show the origin of the genital gland. 500x. 



Fig. 33. Similar section of an older larva, to show the increased development of the genital gland. 500x. 



Fig. 34. Similar section of a still older larva, to show first appearance of lumen and covering epithelium of the genital 

 gland. I, beginnings of lumen; epi, outer epithelium of gland, formed secondarily from right side of mesentery. 600x. 



Fig. 35. Similar section of a young twelve-tentacled larva, to show the genital gland well developed on the right-hand 

 side of mesentery and confined to that side. 500x. 



Fig. 36. Part of a similar section of an adult, to show the formation of the lumen of the genital duct from the lumina of 

 the glands. 200x. 



