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been transformed into an elastic supporting organ, the 
notochord. In the tentacles also of Cnidarians we see the 
endoderm produce large vesicular cells constituting a sup- 
porting tissue which here, truly, has not the elasticity of 
the notochord. Thus l think the notochord is a derivative 
from the endoderm in the same way as the mesoderm but 
of phylogenetically younger origin than the latter. In onto- 
geny also do we see that the development of the mesoderm 
bands in Chordates begins and is completed earlier than 
is that of the notochord. From this circumstance GOETTE 
(1890, p. 26) concludes: “dass die Chordabildung erworben 
wurde, nachdem die bilaterale Mesodermbildung bei den 
Vorfahren der Chordaten bereits bestand.” It is difficult 
to say whether we should consider the notochord as meso- 
derm or endoderm. 
Gill-slits. — As regards the gill-slits, we look once more 
in vain for an homologous structure in Annelids. In the 
chapter on the head, however, | have compared them with 
the paired diverticula of the gut in flatworms and nemer- 
tines. For this l refer to p. It will be evident that the 
gill-slits of Balanoglossus, formed from the ectodermal stomo- 
daeum, can have no connection with those of Vertebrates 
and can only be considered as analogous structures. The 
Deuterostomia can not give us the key to the solution of the 
problem of the origin of Vertebrates and to the explanation 
of the structure of their organs. 
Dr. H. C. DELSMAN. 
