224 F. M. BALFOUR. 
as well as of the nearly universal derivation of the mesoblast 
from the hypoblast. The cases of this kind may be due to 
some merely embryonic changes and have no meaning in 
reference to the adult condition, but I think that we have no 
right to assume this till some explanation of the embryonic 
can be suggested. 
For vertebrates, I have shown that in Selachians the body 
cavity at first extends quite to the top of what becomes the 
muscle plate, so that the line or space separating the two 
layers of the muscle plate (wide Balfour, ‘‘ Development of 
Elasmobranch Fishes,” ‘ Quart. Journ. of Micro. Science’ for 
Oct., 1874. Plate XV, fig. 11 a, 11 5, 12 a, mp.}is a portion 
of the original body cavity. If this is a primitive condition, 
which is by no means certain, we have a condition which 
we might expect,in which both the inner and the outer wall 
of the primitive body cavity assists in forming the muscular 
system of the body. 
It is very possible that the formation of the mesoblast as 
two masses, one on each side of the middle line as occurs in 
Selachians, and which as I pointed out in the paper quoted 
above also takes place in some worms, is a remnant of the 
primitive formation of the body cavity as paired outgrowth 
of the alimentary canal. This would also explain the fact 
that in Selachians the body cavity consists at first of two 
separate portions, one on each side of the alimentary canal, 
which only subsequently become united below and converted 
into a single cavity (vide loc. cit., Plate x1v ; fig. 8 b, pp.) 
In the Echinoderms we find instances where the body 
cavity and water-vascular system arise as an outgrowth 
from the alimentary canal, which subsequently becomes con- 
stricted off from the latter (asteroids and echinoids), together 
with other instances (ophiura, synapta) where the water-vas- 
cular system and body cavity are only secondarily formed in 
a solid mass of mesoblast originally split off from the walls 
of the alimentary canal. 
These instances show us how easily a change of this kind 
may take place, and remove the difficulty of understanding 
why in vertebrates the body cavity never communicates with 
the alimentary. 
The last point which I wish to call attention to is the 
blastopore or anus of Rusconi. 
This is the primitive opening by which the alimentary 
canal communicates with the exterior, or, in other words, the 
opening of the alimentary involution. It is a distinctly 
marked structure in Amphioxus and the Batrachians, and is 
also found in a less well-marked form in the Selachians; in 
