MUSEUM OF COMrxVRATIVE ZOOLOGY. 263 



being represented by the distal border. In the proximal region of the 

 primitive hydi'ophyllium we find that the epiblast and hypoblast have 

 separated from each other, and that between them has formed a layer 

 or cellular mass representing the great gelatinous mass of the medusa 

 bell. Outside of it is the epiblast, while lining the cavity is the hypo- 

 blast. The latter layer can be traced from the lining of the primitive 

 cavity for some distance over the sui-face of the yolk cells under the 

 epiblast. The epiblast can also be traced from the superficial posi- 

 tion on the yolk over the surface of the hydrophy Ilium. I find by a 

 comparison of this figure with those by Metschnikotf representing the 

 first appearance of the float, that it most closely approaclies his Fig. 6, 

 PI. VIII. In his figure, however, we miss a representation of the inner 

 hypoblast between the bud which forms the float and the yolk cells 

 which were seen in the stages here figured. Comparing, however, his 

 Fig. 5 of the same plate with his Fig. 6, we find in the latter an 

 ectodermic bud but no hypoblast, while in the former a layer continuous 

 with the lining of the primitive cavity lies under the epiblast where the 

 float is developed. His Fig. 6 represents the origin of the float as far 

 as the epiblast goes like mine, but we miss in it a deeper layer of hypo- 

 blast which is probably present. The epiblast at this stage probably 

 divides into a superficial and a deeper portion. It is suggested that the 

 latter is the same as the middle or gelatinous layer of the medusa bell. 



Under the visor at the distal rim of the helmet-shaped hydrophyl- 

 lium of Fig. 13 the layer of epiblast is thicker than in most other 

 regions, and has a reddish color. Its surface is rough by reason of ele- 

 vations, which are probably superficial, uprising from the epiblast. At 

 this point, or near by, the serrated hydrophyllia (ser. hi/ph.) character- 

 istic of the second larval stage of Agalma first appear. The diameter 

 of the primitive hydrophyllium from distal to proximal border is 

 .40 mm.; its elevation above the yolk, .15 mm. The diameter of the 

 egg is .47 mm. These larvae were picked out of the water in which 

 the Agalmata were confined at six o'clock, August 8th. The hydro- 

 phyllium naturally floats downward in the water, the yolk being appar- 

 ently lighter. 



Primitive Larva. — The maximum development of the primitive or 

 larval hydrophyllium is reached in the next stage, represented in Fig. 14. 

 In this larva the yolk of the egg is still spherical, and little reduced. 

 in size, notwithstanding the enormous growth of the scale from it. The 

 helmet-shaped hydrophyllium almost completely invests the ovum. The 

 bounding planes of the irregular polygonal cells of segmentation are 



