MUSEUM OF COMPARATIVE ZOOLOGY. 263 
being represented by the distal border. In the proximal region of the 
primitive hydrophyllium 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 surface 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 hydrophyllium. I find by a 
comparison of this figure with those by Metschnikoff representing the 
first appearance of the float, that it most closely approaches his Fig. 6, 
Pl. 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. hyph.) 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, .15mm. The diameter of the 
egg is .47 mm. These larvee 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 
