THE DEVELOPMENT OF ASTERINA GIBBOSA. ie 
gland and neighbouring organs. We see that the madreporic 
pore has commenced to be divided into two by the ingrowth of 
a fold. It is not the case in Asterina, as far as I can make 
out, that the numerous pore-canals found in the fully grown 
adult are derived from fresh perforations, as Cuénot has stated 
(3). Rather the statement which he quotes from Perrier seems 
to give the actual method of their formation.1 We see that the 
openings of the stone-canal proper and the pore-canal into the 
axial sinus are still maintained. The ovoid gland with its core 
is seen to reach right down to the oral end of the axial sinus, 
and to be attached to its oral wall. Embedded in the septum 
dividing the inner perihemal ring-canal (lower end of the axial 
sinus—see woodcut 1) from the perihemal spaces proper is 
the so-called oral blood-ring (sang. cire.). This is a ring-shaped 
tract of peculiarly modified connective tissue; the section shows 
that it is of a different nature from the ovoid gland, and has no 
connection with it. In Asterias this ring gives off radial pro- 
longations traversing the longitudinal septa of the radial 
perihemal canals, but these do not exist in Asterina. The 
development of this structure as far as its histology is con- 
cerned is shown in PI. 24, figs. 107—109, which represent 
small portions of sections parallel to the disc. The first two 
sections are taken from the same specimen as figs. 82—84; in 
this specimen as we have already learned (see above, p. 366) 
the metamorphosis has just concluded. We see that the 
mesenchymatous tissue between the outer and the inner peri- 
heemal rings has undergone differentiation. Most of it has be- 
come converted into fibrous tissue, but at one level no fibres 
have been formed, the whole of the mesenchyme cells becoming 
ameebocytes (sang. circ.); this part is the rudiment of the 
blood-ring. In fig. 109, taken from a specimen in which R 
equals ‘45 millimetre, we see that the ring is completely formed ; 
1 Durham, in a paper on “ Wandering Cells in Echinoderms” (‘ Quart. 
Journ. Mier. Sci.,’ vol. xxxiii), has described the communication of the axial 
sinus and stone-canal in a young Cribrella. He also insists that we have no 
blood-vessels, but rather “ haemal strands ” in Echinoderms, but makes the 
common error of supposing the ovoid gland to belong to this category. 
