376 E. W. MACBRIDE. 
in continuity with the original rudiment of the primitive 
germ cells. 
It is, then, not quite correct to speak of the genital rachis as 
being an outgrowth from the ovoid gland, as Cuénot 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. Cuénot 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 Haman 
(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 
Cuénot (8), 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 celom. At this spot the right (aboral) celom breaks 
through into the left (oral) ccelom, 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 amebocytes of the ccelomic fluid are formed. 
The genital rachis gives off, as it passes each interradius, 
two branches enclosed in corresponding branches of the aboral 
