134 Papers from the Department of Marine Biology. 
develops a knob-like tip (fig. 7) which spreads laterally until it meets 
and fuses with those of the adjoining rays. Gradually a wheel-like 
structure is formed (fig. 9). Figure 8 represents one at a slightly 
earlier stage. 
From the center of this structure there has already grown out in 
each direction a calcareous rod in the position of an axle (figs. 7, 8). 
The one which points internally sends out at its tip a mass of irregular 
branches which lie in a plane parallel to the wheel (figs. 8, 9, 11, sp.), 
and which unite to form a small plate below the spine (figs. 12, 13, 14, 
sp.). The rod that has grown out from the center of the original 
plate now fuses with the base of this structure (fig. 13), so that the cal- 
careous part of the spine is united with the permanent plate of the 
developing embryo. 
The rod which pushes out externally from the wheel divides into 6 
branches that grow up into a cup-shaped structure (fig. 11); each of 
these now sends out externally 3 branches, the lowest of which bends 
downward and joins the wheel at the end of a spoke (fig. 13, 63). The 
uppermost of the three grows downward to meet the second and it in 
turn sends down a branch to meet the third. Meanwhile each of the 
original 6 rods has begun to elongate and to send out branches right and 
left at regular intervals, which meet those of the adjoining rods. This 
forms the lattice-like skeletal structure in the developing spine (fig. 16). 
THE AMNIOTIC CAVITY. 
In the younger of the two stages studied, the embryo forms but a 
small part of the pluteus, while the amniotic cavity, crowded with the 
developing spines, occupies the greater portion. Just how this cavity 
has developed it is impossible to state, since I possess no stages young 
enough to show it in the process of formation, but it seems possible 
that it has grown in from the ventral surface by a process of involution, 
gradually pushing back the gut which may already have lost its con- 
nections with the exterior. Figure 20 shows a median sagittal section 
of such a pluteus. The external opening leads directly into the amni- 
otic cavity, which at this stage is filled with about 15 spines in various 
stages of development. This opening apparently does not close over, 
since it is still present in the 55-hour stage when the spines are now 
pressing closely against the exterior wall of the amniotic cavity. There 
is no indication of the formation of a mouth. 
In correlation with the unusual position of this cavity, the echinoid 
rudiment, instead of occupying one side of the pluteus as in other 
described forms, now becomes pressed back to a central position with 
the amniotic cavity surrounding all but its aboral surface, so that the 
dorsal surface of the pluteus forms the aboral surface of the adult. 
The opposite wall of the embryo, separating it from the amniotic 
cavity, becomes the oral region, and in it the permanent plates develop, 
