. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. ANATOMY AND DEVELOPMENT. 225 ^ by subsequent extension the lumen of the subneural gland and its duct. Thus the anterior portion of the primitive neural tube, having become constricted off from the cerebral vesicle of the larva, and having given rise by proliferation from its dorsal wall to the definitive ganglion, becomes bodily converted into that structure which we shall call, in agree- ment with JuLiN, the hypophysis. The opening of the latter into the base of the buccal tube becomes the dorsal tubercl
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. ANATOMY AND DEVELOPMENT. 225 ^ by subsequent extension the lumen of the subneural gland and its duct. Thus the anterior portion of the primitive neural tube, having become constricted off from the cerebral vesicle of the larva, and having given rise by proliferation from its dorsal wall to the definitive ganglion, becomes bodily converted into that structure which we shall call, in agree- ment with JuLiN, the hypophysis. The opening of the latter into the base of the buccal tube becomes the dorsal tubercle of the adult. Finally, at a much later stage, the glandular portion of the hypophy- sis arises by proliferation of spongy tissue from the ven- tral wall of that portion of the neuro-hypophysial tube which lies immediately be- low the ganglion. A section through the cerebral vesicle of a larva of Distaplia, a colony-build- ing Ascidian, showing the hypophysis in process of â. ^ t^ , , .⢠,k ^, J ^ ^ J ^ Fig. 106. â Frontal section through being constricted off from cerebral vesicle of a larva of Distaplia , . , . . .^. inagnihirva, to show the origin of the the vesicle, is given m hlg. ganglion and hypophysis. (After HJORT; 106. In this genus the con- <=°â¢'^i';^'',°" °f'"â °.'^S",<:=-\ , , ^ In the larva of Distaplia, the hypophy- dition of things generally is sis opens into the branchial sac be- ,._- - , hind tlie stomodo?um. very different from what ^â Cerebral vesicle, ec. Ectoderm. obtains in Ciona, but it is ''"⢠Endoderm. ^4-. Ganglion, hy. Hy- pophysis (neuro-hypophysial tube). introduced to show the essential similarity in the mode of origin of the hypophy- sis in this form, as observed by Dr. Johan Hjort. In Distaplia, as is also the case to a less extent in Clavclina, the ganglion begins to develop from the wall. Please note that these images are extracted from scanned page images that may have been digitally enhance
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1894
