18 EMBRYOLOGICAL MONOGEArHS. 



19. Gastrula three days old. The diverticulum of the gastrula cavity begins to be formed on the left and right 



sides, a shows the left diverticulum. 



20. Longitudinal section of a gastrula soon after its escape from the egg ; the left and right enterocojlum pouches 



are indicated. 



21. The same stage as fig. 20, seen from the left side : L m, the invagination of the ectoderm which eventually 



forms the larval mouth. 



22. The same stage a.s fig. 20, seen from the right side. 



23. Embi-yo at the beginning of the fifth day, seen from the left, a, point of .separation of the gastrula cavity and 



the enterocfelum ; b indicates the position of the gastrula mouth, Gm, which has now disappeared. 



24. Transverse sei'tion of an embryo in the stage of fig. 23. 



25. Longitudhial section of an embryo at the end of the fourth day : in which the g.a.strula cavity a is still in 



communication with the enterocojlura. 

 2(i. Longitudinal section of an embryo at the end of the fifth day; the communication between the gastrula cavity 

 and the enterocoelum has become completely shut olf at a. 



27. Larva of the seventh day, seen from the left side ; the intestine, of which the position is merely indicated, has 



been removeil, to show the mesentery, M, placed behind it. The arrow indicates the communication be- 

 tween the enteroecelum of the larval organ and the enteroccelum surrounding the intestine. 



28. Larva in the same stage as fig. 27, from the anterior side : the position of the intestine is only indicated. 



29. 30, 31. Different views of a larva on the si.xtli day. 



29. Seen from the left side : a, the larval organ (the brachiolarian appendages^ ; h, its anterior, c, its posterior 



lobe ; d, the button-like projection on the creeping surface of the larval organ. 



30. The same as fig. 29, seen from the anterior and left side. 



31. The same, seen facing the creeping surface of the larval organ. 



32. Larva of the sixth day, seen from the left. The hyuroccelum has become five-lobed, 1, 2, 3, 4, 5, its five lobes ; 



the position of the mesentery on the other side of the digestive cavity is indicated as if seen through it. 

 The arrows indicate the communication between the larval entcrociclum, the hydrocaduni, and the entero- 

 ccelum surrounding the digestive cavity. 



33. Larva of the seventh day. The'hydrocoelum is apparently shut off at a from the larval organ ; communica- 



tion between it and the larval organ, however, is still clearly to be made out, and is indicated by the 

 arrow ; b, formation of an ambulacral lobe (1) in the mesoderm below the hydrocoelum ; c, a band of con- 

 nective tissue, not always found in this position. 



34. Larva of the seventli day, from the posterior side. L s, the larval cesophagus ; .S's, pouch of the digestive 



cavity extending towards the hydroccelum, which later trends towards the cesophagus of the young Starfish. 

 The madreporic canal is abnormally early developed. 



35. Exterior view of the satne larva, showing on an upper plane the dorsocential plate, C, with intcrradials 



//ij, JRit JI's, JUi' JR^t and one of the terminal plates, T^ ; on the next somewhat lower plane are the 

 terminal plates T-^, T.^, T^, and on a still lower plane, T.^. 



36. Longitudinal section through the same larva, seen from the anterior side ; a shows the broad open communi- 



cation between the enteroctclum of the larval organ and the hydroccelum, as well as the communication 

 between the dorsal pore and the enteroccelum of the larval organ. 



37. View of a whole embryo, seen from the right and lower side. 



38. View of a larva partly cut open, seen from the right side. Bl", central blood system, a, mesoderm plate 



covered by the entoderm of the enterocoelum ; this has taken the place of the larval cesophagus, which has 

 now disappeared. 



39. A larva of the eighth day, seen from the left side. 



40. A similar larva, creeping, a and b in both these figures denote the anterior and posterior larval lobes. 



41. The surface of the larval organ of an embrj'o somewhat more advanced than figs. 39, 40. 



42. Larva in about the same stage as fig. 41, seen from the right side. 



43. Larva on the eighth day, seen from the right side, showing the position of the blood c:ivity, Bl, in the mesen- 



tery adjoining the fifth interradia], JI!^, in the position where the madreporic plate will eventually be 

 formed. 



44. Larva at the begiiming of the ninth day, seen from the left. 

 4,5. Larva at the end of the eighth day, seen from the anterior side. 



46. Larva of the ninth day, cut open close to the surface on the left, to show the shape of the hydroccelum lobes, 



their unequal development, and the formation of the first and second ambulacral plates. 



47. Larva of the tenth day, seen from the anterior side. 



48. The same, from the anterior and left side. 



49. Larva of the tenth day. This larva still retains a large larval organ ; seen from the left, the section is so 



made as to show the relation of the live lobes of the ambulacral and antiambulacral areas. 



