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rachis, and the space into which it is growing, which is seen to be a | 
diverticulum of the coelom, is the rudiment of the aboral | 
sinus. 
Cuénot3 has made the statement, that the genital rachis is an out- 
growth of the dorsal organ. This however he did not observe; he only 
inferred it from the fact, that the dorsal organ exists before the genital 
rachis, and that the cells in both are similar. Hamann? has described 
the relation of the genital organs to the rachis, and I can only con- | 
firm his results. As it passes each interradial fold of the body wall it | 
gives off two branches which run down the sides of the fold, and end 
in slight enlargements. These in later stages increase very much in 
size, pushing before them, as they project into the body-cavity a por- 
Fig. 2b. 
ARIE madreporie Pore 
Kb... sinus C 
(axial sinus) 
Fly... sinus 6 
A VA (cavity of dorsal organ.) 
AR — RASE mesentery 
sinus a «°° 3 
incipient. genital rachis 
(aboral sinus) 
tion of the aboral sinus. Later as described by Cuénot in the paper 
referred to above, a constriction appears, tending to interrupt the 
continuity of the portion of the aboral sinus surrounding the young | 
genital organ with the portion surrounding the branch of the rachis. 
Thus far we have traced the genital cells back to the epithelium 
lining the canal, which represents the most aboral portion of the dor- 
sal organ. Whence however comes the dorsal organ? 
If we make sections of a very young Asterina which still retains 
a considerable rudiment of its larval appendage, we see that the 
cavity of the dorsal organ is in communication with the coelom, 
and hence its epithelium is peritoneal epithelium. 
3 Cuénot, » Contribution a l'étude anatomique des Astérides« Archives de 
Zoologie Expérimentale. Tome Vhis supplementale. 
4 Hamann, » Die wandernden Urkeimzellen und ihre Reifungsstätte bei den 
Echinodermen.« Zeitschr. f. wiss. Zool. 46. Bd. 1887. 
