258 E. W. MACBEIDE. 



in continuity with the original rudiment of the primitive 

 germ cells. 



It is, then, not quite correct to speak of the genital rachis aa 

 being an outgrowth from the ovoid gland, as Cuenot has done 

 (3) . This statement, nevertheless, marked a step in advance in 

 our knowledge, for it gave a hint as to the meaning of the ovoid 

 gland. Cuenot found specimens of Astropecten with the ovoid 

 gland, but without the genital rachis, and noting the identity of 

 the character of the cells in the two structures, stated that the 

 rachis was an outgrowth from the gland, though he found no 

 intermediate stages. These were first found by me (14) in 

 the Ophiurid Amphiura squamata, and at the same time I 

 demonstrated the epithelial origin of both gland and rachis. 

 It is the genital rachis which of course was formerly known as 

 the aboral blood-vessel; in most Asterids and Ophiurids it 

 later undergoes partial degeneration, giving rise to cells con- 

 taining violet pigment. Ludwig, however (11), and Hamann 

 (7) have pointed out that the central core remains unaltered ; 

 the latter was the first to point out that in all Echinoderms, 

 except Holothurids, a genital rachis exists, of which the 

 genital organs are local outgrowths. In Amphiura squa- 

 mata, however, and in Asterina gibbosa, according to 

 Cuenot (3), the whole genital rachis remains unaltered through 

 life ; this is only one of the many points in which Asterina 

 shows itself to be one of the most primitive of Asterids. In 

 the plans given in text-books of the blood system, two vessels 

 are shown proceeding from the aboral ring in the interradius 

 of the madreporite to the pyloric sac. These are two mesen- 

 teric bridles, remnants of the piece of septum left at this level 

 between the two horns (right dorsal and right ventral) of the 

 left coelom. At this spot the right (aboral) coelom breaks 

 through into the left (oral) coelom, perforating the piece of tissue 

 referred to, and leaving only the mesenteries. The peritoneum 

 covering them seems to be peculiarly modified, and is possibly a 

 place where the amoebocytes of the coelomic fluid are formed. 



The genital rachis gives off, as it passes each interradius, 

 two branches enclosed in corresponding branches of the aboral 



