32 MORPHOLOaY OP 



(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 as a fine tube. In 

 later life a great thickening of the notochord takes place at this 

 point, and the lumen then acquires a downward extension 



