262 E. W. MACBRIDE. 



Cueaot brings to show that they escape by diapedesis into the 

 axial sinus is quite insuflficient. The cells of outer epithelial 

 lining are not flattened but cylindrical, and I strongly suspect 

 that he has mistaken their freely projecting ends for escaping 

 amoebocytesj and I may remark that this curious outer 

 epithelium shows its distinctive character from the time the first 

 rudiment of the ovoid gland appears. Whatever its function 

 may be now, there is no doubt that the ovoid gland was pri- 

 mitively a part of the genital organ, and probably is a remnant 

 of the arrangement of the reproductive cells before the radial 

 symmetry was acquired. It is interesting to notice that it 

 originates from the left posterior coelomic wall, whereas an 

 analogous organ in Crinoids arises in the right or aboral coelom, 

 so that they are not strictly homologous. 



If Hamann is, as there is strong reason to suppose, right 

 in stating that the primitive germ cells wander along the 

 rachis into the genital organ, it seems very probable that, 

 at any rate in the young adult, the ovoid gland is a centre 

 of formation of the primitive germ cells; and its relation to 

 the axial sinus may have to do with its aeration, for it must be 

 remembered that the pore-canal opens into the axial sinus, and 

 the current in this is, as we shall see, inwards. lu the fully 

 grown adult it no doubt undergoes, to some extent, the degene- 

 rative change noted above in the genital rachis of other genera. 

 What the meaning of this change is, is very obscure. Obser- 

 vations on the histology of the gland at different seasons might 

 elucidate its meaning. 



Turning now to the stone-canal, we see, in fig. 118 (a section 

 transverse to the axial sinus and stone-canal), the beginning 

 of that curious T-shaped ingrowth which is so marked a feature 

 of the stone- canals of Asterids, but which is much less developed 

 in Asterina than in other genera. It is covered by short cilia, 

 the rest of the epithelium bearing long flagella. 



Cuenot asserted that the stone-canal was a functionless rudi- 

 ment, the current being neither outwards nor inwards. Ludwig^ 



' Ludwig, " Ueber die Function der Madreporenplatte und des Steinkanals 

 del- Echinodermen," ' Zool. Anz.,' 1890, p. 377. 



