Plate III. 



a: mouth, 

 b: gullet. 



c: vaso-peritoneal vesicles. 

 d: stomach, 

 e: intestine, 

 f: anal aperture. 



g: opening between the stomach and the intestine. 

 Fig, 37. Young Gastrula at twenty-nine hours after the fecundation. Optical section. Length = 

 0,22 mm. Breadth = 0,u mm. 



p 38. Gastrula at about forty hours after the fecundation; it has just begun to change into a 

 Pluteus. Optical section. Length = O.24 mm. Greatest breadth = 0,i4 mm. 



T> 39. Side view of a young Pluteus at about forty-eight hours after the fecundation. Length 

 = 0,24 mm. 



>) 40. Young Pluteus somewhat more advanced, at forty-eight hours after the fecundation or 

 slightly more. Seen from the ventral surface. Length = 0,22 mm. Hardened with 

 osmic acid and stained with Beale's carmine. 



B 41. The same Pluteus. Side view. 



ji 42. Diagram of the archenteron in a very young Pluteus. The shaded area marks the posi- 

 tion of the blastopore. 



n 43. The same in a slightly more advanced Pluteus. 



B 44. The same in a somewhat older Pluteus. showing the oral invagination, the different por- 

 tions of the intestinal tract and the vaso-peritoneal vesicles, which last are almost con- 

 stricted off. 



» 4.5. The same slightly later. The oval invagination has joined the archenteron and the vaso- 

 peritoneal vesicles have separated. 



B 46. The same. Side view. 



B 47. The blind end of the archenteron and the vaso-peritoneal vesicle at the beginning of 

 their separation. 



B 48. The same in a slightly older larva. 



B 49. The same in a more advanced larva. 



B 50. Longitudinal section through ïhe vegetative pole of a Blastula, to show the separation of 

 wandering cells. Treated with Flemming's fluid. 



» 51. The same of another Blastula. Hardened in Flemming's fluid and stained with hsematoxylin. 



» 52. Longitudinal section through the invaginating pole of a very young Gastrula. Treatment 

 the same. 



B 53. The same of a slightly older Gastrula. Treatment the same. , 



B 54. The same of a somewhat older Gastrula. Treatment the same. 



» 55. Longitudinal section through the blind end of the archenteron of an older Gastrula. 

 Treatment the same. 



» 56. Longitudinal section through the archenteron of a Gastrula. Treatment the same. 



» 57. The same. Treatment the same. 



