ANATOMY AND DEVELOPMENT. 22/ 



Ascidians and in Amphioxus. Fortunately there is one 

 structure as to whose complete homology, in the Urochorda 

 (Tunicata), on the one hand, and the Cephalochorda, on the 

 other, no one entertains a doubt, and that is the endostyle. 

 We thus have in the endostyle a firm basis upon which to 

 ground our deductions. 



In the larva and in the young Ascidian before the 

 primary long axis has been disturbed in the way which we 

 shall shortly describe, the endostyle is the most anterior 

 endodermic structure in the body, and lies dorso-ventrally 

 at right angles to the long axis of the body (Fig. 105 A 

 and B, c}. 



As described above in the larvas of Amphioxus, particu- 

 larly in the younger larvae (see Figs. 64 and 73), the endo- 

 style, though lying asymmetrically on the right side, being 

 involved in the general asymmetry of the larva, is quite 

 anterior in position, in front of all the gill-slits and partly 

 in front of, though also partly opposite, the mouth (on 

 account of its asymmetry), and almost at right angles (see 

 especially Fig. 64) to the long axis of the body. As there 

 is only a short stretch of simple endoderm in front of the 

 endostyle in the larva of Amphioxus, we may describe it 

 as the most anterior differentiated endodermic structure 

 in the larva, thus corresponding with remarkable precision 

 to the condition described above in the larval and newly 

 fixed Ascidian. 



In the middle of the wall of the branchial sac in Fig. 

 105 B are seen, somewhat in front of and below the atrial 

 aperture, a, of this side, two lens-shaped structures whose 

 slightly concave sides face each other. These are the 

 borders of the two first-formed primary branchial stigmata 

 or gill-clefts. Their actual openings into the atrial chamber 

 are at present so small that they can hardly be seen in 



