30 DE, J. P. GEMMILL ON THE DEVELOPMENT OF 



the hydrocoele, the dorsal sac, the epigastric coelom, and the preoral ccelom. The 

 structures whose development we have thus to describe are : the internal oral circular 

 sinus, the perih^mal pouch of interradius IX/I, the axial organ, and the stone-canal. 

 We have also to indicate what are the final relations of the axial ccelom and the stone- 

 canal to the pores of the madreporic plate. 



Internal Oral Circular Sinus. — This sinus is derived partly from the axial ccslom and 

 partly from the left lateral diverticulum. It will be remembered that the hydrocoele 

 ring is nipped off from the bottom of this diverticulum, the rest of the diverticulum 

 being still unappropriated and opening widely into the axial ccelom. From the latter 

 cavity, indeed, it is now no longer to be sharply distinguished, since at the commence- 

 ment of the fixed stage, while the preoral lobe is settling back on the left (larval) side, 

 the axial part of the anterior ccelom also undergoes a certain amount of migration 

 backwards and to the left. At a later stage in metamorphosis the wall of the enteron 

 lays itself against that of the oral surface in preparation for the formation of the mouth. 

 In this process the remainder of the left lateral diverticulum undergoes obliteration of 

 the central part of its cavity, as well as of that segment of its periphery which is in 

 the neighbourhood of radial pouches III or IV and V of the hydrocoele. The rest 

 goes to aid the axial coelom in forming the internal oral circular sinus, which at this 

 stage is naturally incomplete in the region of the pouches just named. The com- 

 pletion takes place later by extension of the sinus-cavity from either side, and this 

 extension seems to be aided by the fact that two or three small lacunae have 

 been left along its track during the process of obliteration referred to above. The 

 internal oral circular sinus now forms an annular cavity surrounding the mouth inside 

 the ring-canal of the hydrocoele. A ribbon of haemal tissue comes to grow round it in 

 the manner described below under " Axial Organ." 



The Axial Organ. — The axial organ forms a fold of the wall of the axial ccelom 

 running parallel with the stone-canal on the dextral * side of the latter, as one looks 

 from the aboral aspect. As stated above, the cells invaginated into the dorsal sac are 

 continuous with those of the axial organ. The first appearance of the organ is due to 

 proliferation in situ of a strip of cells belonging to the lining of the axial coelom. A 

 ridge is thus formed consisting of cells rich in protoplasm, rounded or polygonal in 

 shape, and with very little intercellular substance. The organ is relatively large in the 

 middle or later stages of metamorphosis, and even then can be traced oralwards into a 

 ribbon of similar tissue extending for some distance along the wall of the internal oral 

 circular sinus in the sinistral * direction as viewed from the oral side. This ribbon 

 afterwards becomes the circular haemal vessel, which in the adult lies in the septum 

 between the internal and the external oral circular sinuses. 



The genital rachis grows through the upper part of the axial organ (p. 36), extending 

 downwards at the same time for a short distance, but not nearly reaching to the 



internal oral ring. 



* See note on p. 13. 



