THE ANCESTRY OF THE CHORDATA. 565 
mesoblast already arose thus, and when new bundles of 
muscles formed in the adult began to arise in the larva they 
arose in the same manner as the primitive mesoblast. That 
provision is made for the production of more mesoblast than 
that of the original fourteen pairs of pouches is shown by the 
presence of mesoblastic pole-cells in Amphioxus (Hatschek). 
In any case the existence of Balanoglossus proves that the 
notochord, gill-slits, and Chordate nervous system were present 
together before the myotomes were formed. 
The Gill-slits.—It is unfortunate that the facts of the 
Enteropneusta seem to throw no new light on the original 
meaning of gill-slits. That they do not do so tends, however, 
to show that probably gill-slits were from the first developed 
as such, and not as modifications of any previously-existing 
organ, as has been sometimes held. 
The folded skeletons of the gill-shts of Balanoglossus are re- 
markable in their resemblance to those of Amphioxus. Until the 
development of these latter is fully known no further com- 
parison can be instituted. It is clear from their origin in 
Balanoglossus that no “ myotomes” are obliterated between 
them (as has been suggested by some, with the hope of in- 
creasing the symmetry of the body), for plainly their repeti- 
tion preceded that of the myotomes. 
The Excretory System. 
Upon the origin of the excretory system of Vertebrata 
nothing can be affirmed from a study of Balanoglossus. The 
excretory systems of Vertebrata cannot be easily derivable 
from anything found in either Balanoglossus, Ascidians, or 
Amphioxus. The absence of any reguiar excretory system in 
these three forms may, perhaps, be correlated with the extraor- 
dinary development of their respiratory systems, which may 
possibly assist in this function. The one fact which is de- 
rivable from the morphology of Balanoglossus, Ascidians, and 
Amphioxus, is that it is nearly certain that the excretory 
system of other Chordata has been developed within the group. 
The Pituitary Body and Proboscis Pore.—Though 
