VOL. IIl.] Balanogtossus. | Igt 
cartilaginous sheath, which is directly continous with a corresponding 
sheath surrounding the notochord, while in Balanoglossus no such 
’ sheath is found, the nerve cord and notochord not only not being 
in close relation, but the dorsal blood vessel is situated between 
them. However, so far as the absence of the sheath is concerned, 
the difficulty is hardly a weighty one since we must suppose, both 
from developmental evidence and on a friorz grounds, that the 
earliest vertebrate ancestors were without such a sheath. But the 
situation of the dorsal blood vessel as described is not so easily ex- 
plained away, though Dr. Morgan‘ has suggested that the dorsal 
aorta of vertebrates is another vessel entirely. His suggestion 
would seem to imply that in vertebrates the dorsal aorta has arisen 
since the vertebrate phylum branched off from the common ances- 
tral form, and that the dorsal vessel corresponding to the one now 
found in Balanoglossus has disappeared. This conjecture may re- 
ceive support from the fact that the heart of Balanoglossus is situated 
in the proboscis, and hence cannot certainly have any relation to the 
vertebrate heart. 
In this connection it seems to me worth while to refer to the 
lymph canals described by Lankester* within the notochordal 
sheath, one on the dorsal side and one on the ventral side, in Am- 
phioxus. And the same author speaks of the great difficulty in 
distinguishing blood vessels from lymph vessels in this animal. It 
would be rash to maintain a homology between the lymph canal in 
the dorsal portion of the Amphioxus notochord and the dorsal blood 
vessel of Balanoglossus, yet no harm can come from a cautious sug- 
gestion of such a possibility. 
The notochord of Balanoglossus originates from the dorsal wall 
of the digestive tube as it does in vertebrates, and in later stages of 
development resembles the vertebrate notochord in its histological 
structure considerably, thus satisfying two of the important criteria 
of homologous structures. But, in all vertebrates, without excep- 
tion, the notochord arises from nearly or quite the entire length 
_of the embryonic digestive tube, while in Balanoglossus it arises as 
an evagination from near the anterior end and grows out anteriorly 
‘T.H. Morgan. Growth and Metamorphosis of Tornaria, Journ. of Morphology, 
Vol. v, p. 407, 1892. ; 
5. Ray Lankester. Contributions to the knowledge of Amphioxus lanceola- 
tus, Yarrell, Quart. Journ. Micro. Sci., Vol. xxix, p. 365, 1889. : 
