18 EiVIBEYOLOGICAL MONOGEAPHS. 



27. Emliryo with sixty-four lenticular cells, forming the epiblastic vesicle, and thirty-two masses, composed of 



transparent protoplasm, the hyix)blast {hb), 

 23. Cell at the beginning of the sixth plane of segmentation, epl. Epi[ilasm. hpl. Hypoplasm. 



29. The epiblastic layer now grows faster than tlie hypoblast, and the space between them increases, while at 



points they grow together. Appearance of the ovum towards thi^ close of the fonnation of the seventh 

 plane of segmeutation. epl. Epiplasm. hpl. Hypoplasm. fi. liridge connecting the two. 



30. Appearance of an ovum a day after fructification, epl. Epiplasm. c b. Epiblast. hb. Hypoblast. Ji jil. Hy- 



jioplasm. nl. Cell nucleus, vt. Vitellus. 



31. The wide cavity between the two layers, epiblast and hypoblast, now becomes filled with a gelatinous layer. 



The embryo becomes ciliated, and at the point where epiblast and hypoblast fuses, the epiblast (e i.) is 

 thickened and forms a disk, through which opens a mouth. Embryo thirty liours after impregnation. 

 e b. Epiblast. c V. Disk-like epiblastic thickening. The disk is seen betweeu the dots (c V). h b. Hypo- 

 blast, vbr. Umbrella, gelatinous tissue of a middle layer. 



32. Em brj-o forty hours after imjiregnation. eft'. Edge of the epiblastic disk. hb. Hypoblast, which fuses with 



the eiiiblast at this point, ubr. Umbrella. The epiblast [c b.) is spread as a thin invisible layer over the 

 surface of the umbrella. 



33. The oral pole of the embryo three days a.nd ten hours after fecundation, e b. Epiblast. or. Thickened epi- 



blast, which later breaks tlirough and forms a mouth, h b. Hypoblast. 



34. Oral pole of an embryo six days and twelve hours old (after fecundation), with mouth widely open, and the 



border of the umbrella ()(?;/•.) drawn somewhat together, tb. Epiblast. hb. Hypoblast. </>. Solid axis 

 of tentacle, f. Conical projection at the extremity of the tentacle. 



35. Older larva with lijjs and umbrella shown fiom below. The tentacles are widely extended, cb. Ejiiblast. 



ga. Stomach, lab. Lips of entrance to stomach cavity, nl cij. Nematocyst. vd. Velum, (p. Connec- 

 tion of the bases of the tentacle with the bell margin. 



36. Oldest larva, which swims for the most part by means of movements of the velum (uc/). No cilia, ga. Stom- 



ach. Zrti. Lip. ji^c?/. Nematocyst. te. Tentacle. The small buds on the rim of the bell indicate the future 

 sense bodies as well as rudimentary tentacles. 7ibi: Undindla. 0. Tentacular axis of solid hypoblastic 

 cells, and the same axis continued from tlie base of the tentacle to the bell margin. 



