THE DEVELOPMENT OP ASTEEINA GIBBOSA. 255 



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 Cuenot has stated 

 (3). Rather the statement which he quotes from Perrier seems 

 to give the actual method of their formation.^ 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 perihsemal ring-canal (lower end of the axial 

 sinus — see woodcut 1) from the perihsemal spaces proper is 

 the so-called oral blood-ring {sang, circ.) . 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 

 perihsemal 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. XVII, 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. 83 — 84; in 

 this specimen as we have already learned (see above, p. 248) 

 the metamorphosis has just concluded. We see that the 

 mesenchymatous tissue between the outer and the inner peri- 

 hsemal 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 

 amoebocytes {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. Micr. 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 tlie 

 common error of supposing the ovoid gland to belong to this category. 



