. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. 1- 'sm Ml 11 -'I' 138 OF AMPIIIOXiS. the region of the secondary slits. Such of the primary slits as occur behind this region are not affected by the rotatory method of growth, and retain their original position in the mid-ventral line of the pharynx. It is to be noted also that there are only twelve primary gill-slits at this stage. Assuming that in the particular larva here figured there were originally fourteen primary slits, the fourt
. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. 1- 'sm Ml 11 -'I' 138 OF AMPIIIOXiS. the region of the secondary slits. Such of the primary slits as occur behind this region are not affected by the rotatory method of growth, and retain their original position in the mid-ventral line of the pharynx. It is to be noted also that there are only twelve primary gill-slits at this stage. Assuming that in the particular larva here figured there were originally fourteen primary slits, the fourteenth has closed up and vanished without leaving a trace, while a vestige of the thirteenth can still be recognised. The actual process involved in the closure and disappearance of a certain number of the primary slits can, as we shall see, be readily observed in the living larva. Cliib-sliapcd Gland and Endosiyle. The internal aperture of the club-shaped gland into the pharynx is exceptionally plain at this stage, and its refring- ent walls and relatively large size give it a curiously slit- like appearance. We shall find that the gland subsequently atrophies, but the most persistent part of it —that is to say, the last part of it to disappear — is precisely the internal opening with its refringent border. The cndostyle, whose primary position, as we have seen, was immediately in front of the club-shaped gland, now presents a remarkable condition. It has begun to grow backwards and downwards, being probably pulled down, so to sjieak, by the general rotatory growth of which we have spoken above; and so the club-shaped gland no longer lies behind it, but upon it. The gland itself being disconnected with the wall of the pharynx, except at its upper end where it opens into the latter, is not affected by the complicated changes to which the pharyngeal wall, including gill-slits, mouth, and cndostyle, is subjected, so. Please note that these images are extracted from scanned
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectfishes, bookyear1894
