112 WILLIAM BATESON. 
(3) a simple branchial skeleton, not connected by longi- 
tudinal bars, as in the other forms ; 
(4) very short collar funnels, the external opening of 
which is directed transversely instead of posteriorly, 
as in the others, in consequence of 
(5) the greater extent of the backwardly-directed atrial 
fold. 
As far as can be determined from Agassiz’ account, his 
species agrees in these points with the one which is the subject 
of this paper. 
The Notochord and Axial Skeletal Rods. 
B. Kowalevskii.—The general course of development of 
the notochord has been already described. Fig. 47 shows the 
histological characters of the cells at two gill-slits, when they 
are still large full-looking cells with large nuclei. 
Fig. 48 is from a section of the proboscis stalk in the region 
of the pore of a larva with three gill-slits. It exhibits the 
commencing degeneration of the notochordal tissue and the 
increase in size of the structureless deposit which constitutes 
the skeletal rods. 
In this region the skeletal rods unite to form a single 
median rod, which is continuous with the notochordal sheath. 
In figs. 49—53 the appearance of the notochord at four gill- 
slits is illustrated. The degeneration is now far advanced. 
Nuclei are rare in the notochord, and the cells are vacuolated, 
as shown by the fact that the nuclei occur in the nodes of the 
cell outlines. The protoplasm of the cells merely forms a kind 
of network containing a few nuclei. The remainder of the 
space is probably occupied by some homogeneous non-proto- 
plasmic substance, such as may be supposed to fill up the 
notochordal tissues of other forms. 
Figs. 49—52 are from sections taken in front of the lumen 
(cp. fig. 57). In fig. 53 the lumen is reached. It will be ob- 
served that the lumen at this point still ends asa fine tube. In 
later life a great thickening of the notochord takes place at this 
point, and the lumen then acquires a downward extension 
