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, 6 3 ). 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 cavit}^ 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, 



