DEVELOPMRNT OF ATEIAL CHAMBER OF AMPHIOXUS. 459 



cells has sometimes the appearance in published dl•a\^ings of 

 larvae — of being a shadow cast by the gland, or in some cases 

 looks like a duplication of it. It can be traced in the section 

 drawn in fig. 2, PL XXX^ where it is marked me. 



In the deep- focus drawings^ PI. XXIX^ figs. 2 and 5, another 

 tubular structure is figured, which is also seen in the transverse 

 sections (PI. XXX, figs. 2 and 3, neph., and fig. 4, neph. a. ; and 

 PL XXXI, fig. 13, neph.). This, so far as we can judge from 

 the drawings given in Leuckart and Nitsche^s diagram, is the 

 structure which Hatschek has described as a nephridium in the 

 'Zoolog. Anzeiger,^ 1884, p. 517, without a figure. In the 

 condition in which we have observed this structure (viz. in 

 larvse I'anging from the stage with three gill-slits up to closure 

 of the atrial cavity) there does not seem to be any special 

 reason for regarding it as a nephridium. We should prefer to 

 call it the subchordal tube. It appears to end blindly ante- 

 riorly, and to open into the buccal cavity near the recurved 

 extremity of the glandular tract which accompanies the club- 

 shaped gland. The tube lies below, and to the left of, the 

 notochord. 



The drawings of the larger larva (PL XXIX, figs. 4 and 6) 

 show some interesting features as to the disposition of the 

 gill-slits and the metapleura. In this larva the atrial tube 

 has formed from behind (the atriopore) forwards as far as the 

 hindermost still very small gill-slits {gs. 9). It is a remark- 

 able fact that all the gill- slits up to this stage originate 

 in the median ventral line. This is true of the first and of 

 all that follow up to the fourteenth, and possibly some few 

 more. It is, however, not true of the formation of new 

 gill-slits after the right and the left lateral series of gill-slits 

 have become established. The figures in our plate show that, 

 whilst gill-slit No. 1 occupies an entirely lateral area on 

 the animal's right side — not reaching below to the median line 

 — this position is gradually receded from by the hinder slits, 

 which from No. 6 onwards are seen to encroach more and 

 more on the left side. When we remember that gill-slit 

 No. 1 as well as all that follow it originated in the 



